I'd like to place these additional CPLDs on a different PCB. This has the advantage that I can simply extend the device when I need. On smaller harnesses, a single 114 test-point PCB will suffice. On larger ones, I can cascade.
There are multiple levels of modularization which you can aim for. Where you want to stop depends on your specific use case. At the most basic level, the hardware must be designed such that you can select the number of modules in use after the design stage. The difficulty of changing the number of modules, space available, desired software complexity (and available space for software, especially on a CPLD) and the system cost will be key factors in your decision.
Hardware
The simplest and cheapest way to do this is to build one PCB, (You don't need multiple PCBs for modular design!) and put footprints for your desired maximum number of CPLDs on the PCB. If you need more IO, you can then solder down another CPLD. Obviously, this isn't something you'd want to do very often.
At the next stage, you'd want to build daughtercards so that you can more easily add and remove modules. You asked:
But what would be the best way to actually connect the PCBs together?
This depends on your system architecture and number of modules. If you know you'll never want more than, say, 3 modules at any one time, just put three connectors on the main board. These can be edge connectors, or stackable connectors, or whatever you like that doesn't require wires. If the number of sub-modules is too large to fit connectors for each on one PCB, then you should consider stacking (if your bus can handle the fanout of your maximum number of modules) or daisy-chaining (if you need to buffer the signal or vertical space is limited) the modules.
Plenty of connectors are designed for this purpose; check the "Board-to-Board" section of your favorite distributor or manufacturer, and many are designed for extremely low crosstalk and high frequency - 500kHz is nothing, unless you're using PTH 0.1" breakaway headers and have fast-changing signals (even then, you're probably OK). Check the mating strength of your connectors just to be sure, but if you only have a few pins, the footprint of your interconnection doesn't carry the stresses well, or the system will be subject to vibration, you'll need standoffs. It's often wise to design the interface in such a way that different modules can be designed to interface with the motherboard in the future. Pins are cheap, give yourself a couple spares just in case!
Software
If your number of modules supports it, you can simply add a slave select line for each module. This isn't really a software solution, but I wanted to mention it.
If you don't mind programming every CPLD differently, you could build the system such that the microcontroller sees it as one giant shift register (which you've suggested). If you added or removed a module, that module's address space would simply be wasted, and extra time would be used transmitting to addresses which don't exist. Each module would need to 'know' its address space, though, which would make programming the complete system a struggle.
A more versatile solution is to use software addressing to access each sub-module. In a 'programming mode' (perhaps a pushbutton on the module, or simply only connecting one at a time), you could assign the CPLD an address. By assigning each CPLD a different address, you could add or remove modules at will, and only have to adjust the activity of the microcontroller (which I presume to be easier to adjust than the CPLD).
My suggestion for this project
If a 324-pin device will solve all your foreseeable use cases, then the single-PCB method should work fine. The multiple-slave-select method would allow you to program all the CPLDs simultaneously with a single programmer. Sorry, but this project as described doesn't seem like a candidate for daughtercards.
That is an excellent guide and seems to cover the relevant points well.
After having read the guide, why do you think that you can cut corners and use simply interconnected pins? It MAY in fact be possible in practice, but it is not obvious that it would be safe or reliable (even though it may be :-) ).
Joining the pins from two SIMS together and powering or enabling one but not the other will result in the unpowered SIM loading the lines to and from the powered SIM and may lead to phantom powering of the "unpowered" SIM.
Note the note on page 17 that says that fig 6 (and so probably fig 7) will not work with 1.8V supply and that the circuit of fig8 is required for use with 1.8V supply.
A multipole mechanical relay would meet the need.
A 4 pole double throw mechanical switch would meet the need if manbual switching was OK. With care less than 4 poles could be used (eg Vcc could be left live with proper attention to levels.)
Best Answer
Possible? Yes. Easy? Not so. You would have to detect the direction within the CPLD and switch the pins between input and output accordingly. It is a lot easier to just use an analog multiplexer chip like the 74HC4051 and control the address bits from a CPLD or microprocessor.
In practice there is no phase relationship between CLK and IO, so it is not an issue. CLK is only used to define the baud rate of the IO line and runs at a much higher speed than the duration on a bit over the IO line. The initial speed of the IO line is one bit per 372 clock ticks. Your GSM modem may negotiate a higher speed transfer later on but lag on the CLK will never be a problem.
If you want to switch the VCC of the SIM cards via the CPLD check the maximum current that the CPLD can drive. You may need an additional driver to provide enough power.
A good opportunity to start lerning how to use them. They're fun!
On the other hand if you want to finish your project fast it is probably easier to just use four analog multiplexers like the 74HC4051 and switch all signals in parallel from a microprocessor. All you need are 3 GPIO pins to control up to 8 SIM cards. You won't have to deal with different voltage levels that way either.
Oh, one last thing: In practice all SIM cards nowadays support 1.8V and 3V so you don't really have to follow the power up sequence of starting with 1.8V and then switching up to higher voltages. For a commercial project I would not recommend this, but for a hobby project I think it's fine to simplify here.