Should work. Cheap charge controllers are a bang bang MOSFET switch, so either the battery is straight on the solar cell, or not. MPPT controllers have a switch mode buck converter; two of them on one panel will get very confused.
Once the battery is full, the charge controller switch will be mostly off, so the other charge controller can use the solar cell.
The danger might be that if both batteries are low, both controllers will come on together, shorting two batteries together and damaging the controllers. You could check for diodes or add them. They must go before the controllers, not between the controller and the battery, as it needs to sense the battery voltage.
The open questions are 1) are there already diodes in the charge controller, and 2) what does the controller do when the solar voltage collapses to ~14V (when the other one shorts the panel to the battery). It might panic and not charge.
It's worth asking the supplier about both of these, I'm sure they've had these questions before
You may be best off using 'conventional' monocrystalline silicon PV cells, arranging Vout to best suit your needand maximising packing density and area occupied on your device. Many manufacturers waste substantial area in intercell spaces (> 10% common, 20% not uncommon) and this can usually be reduced to maybe 5% of area with care.
Optimisation of front sheet optical loss can help make up losses elsewhere. Worst case should be 10% and around 1%-2% is possible if cost is not an issue.
The MCP73871, data sheet here acts as a linear regulator for charging purposes. Max Iin = 1.8A so at 4.2 V out max power = V x I = 4.2 x 1.8 = 7.6 Watts. However, unless you use a preceding MPPT converter the proposed 8V Vmp PV panel will have a maximum efficiency of 4.2/65 = 70%. LiIon cell mean voltage across charging range is closer to 3V7 so mean efficiency ~~ 3.7/6 = 62%.
The LT3652 data sheet here is a nice device if the 7,5V Vstart can be accommodated. Note that the application note tends to focus on battery chemistries which can be floated when 100% SOC (stae of charge) is reached. Lithium Ion cells MUST NEVER be "floated at end of charge. Charge voltage MUST be removed at the end of charging, that their days may be long on the face of the land. The LT3652 can accommodate this need with suitable design.
A simple buck converter / charger such as the LTC4002 may better meet your needs. This allows 5V to 22V input and uses and external MOSFET switch & external flywheel diode and external current sense resistor. fficincy of about 85% can be obtained at Vin = 6V and adding a synchronous rectifier FET in place of the flywheel diode may increase efficiency slightly.
LTC4001 is a 2 A buck regulator LiIon charger BUT Vin max of 5.5V is an annoying limitation.
Your LiIon cell will need at least 4.2V to charge fully - say 4.5V minimum available.
Sunpower cells are difficult to join when cut into fractions of a cell. Most PV panel manufacturers appear unable to do this. Using whole cells should present no difficulty.
Boosting from low voltages is usually less efficient end to end than either boosting from a voltage closer to FVout or buck converting from above Vout.
The SPV1040 is capable of 3 Watt output maximum (data sheet table page 6).
The inductor maximum maximum current (really the switch max current) is shown as 1.8A as you say BUT you must use the worst case = minimum-maximum current for design purposes.
The graphs in the SPV1040 data sheet on pages 8 & 9 show efficiency at various Vin/number of cells combinations.
At full power with 3 cells they show 80% at 4.5V out at 2 Watts without MPPT and 89% with MPPT.
Note that with 3 cells they show 2W max.
Sanity check:
3 cells ~= 1.5V full sun.
Duty cycle at 1.5V in and 4.5V out at 100% efficiency
= 4.5/(1.5+4.5) = 75%.
Max power at 100% = 1.5V x 1.8A x 75% = 2.0 Watts IN.
= what they said.
Pout =~ 2W x 90% = 1.8W out MAX = far short of your desired power level.
For the SPV1040 ILX appears to be the inductor current, & the switch current & the LX pin current.
Best Answer
If the 18 V rating on the cells is the zero current voltage (open circuit voltage) it may not be high enough. Since the cells have some internal resistance, they will be at a lower voltage when there is a load. You may want to consider more than the batteries for the load since you may want to operate the boat and charge the batteries at the same time. If you need a boat that can function no matter what the solar profile, you need to make sure that you can get your batteries charged when conditions allow it, so you need to get as much current out of the solar cells as you can even when you batteries are near full. Some charge controllers have buck/boost converters so you don't have to worry about voltage matching. This allows you to pull as much current as possible, even if it means dropping the voltage on the panels down to 3 or 4 volts. If you are building your own, maybe you don't have this option. In this case, you need to open circuit potential to be as high as possible to get high currents at you battery voltage.