Is it possible to modify a 5 lead unipolar stepper motor to a bipolar

Having just addressed this myself I'd have to say separate drivers for each motor is the most sensible approach.

If you remove the power from one motor, is has no holding torque and will 'flop' under any force.

You may also have issues with the state of the outputs not being correct for the current rotor position, which will make the motor jump a bit when you switch it.

And as Dave mentioned, if you have power on a coil when you switch it, it will arc inside the switch which will eventually destroy it.

It's also way more impressive to have both axes moving simultaneously if you can :)

Your application may be able to cope with some of these issues, but if you need reliable accuracy then I'd recommend separate drivers. The A4988 (update of the A4983) is only $3-4 from Digikey if you're building your own board.

You have a universal stepper: it can be driven in either a unipolar or bipolar stepper configuration. Here is a diagram of the connections for a 6-wire stepper:

You should measure the resistance between each unique pair of wires -- this will be 15 measurements. Put them in a table like this:

     1    2    3    4    5    6
1    X    R12  R13  R14  R15  R16
2    X    X    R23  R24  R25  R26
3    X    X    X    R34  R35  R36
4    X    X    X    X    R45  R46
5    X    X    X    X    X    R56
6    X    X    X    X    X    X

Measure resistance from wire 1 to wire 2, enter that for R12. Measure resistance from wire 1 to wire 3, enter that for R13. ... Measure resistance from wire 2 to wire 3, enter that for R23... until you are done.

You don't need to measure the values in the cells marked X since those values are redundant.

You'll find that there are two center taps on your 6-wire stepper motor, which will have resistance R to the end taps on that coil, and those end taps will have twice the resistance, 2R, to each other. The other coil will have infinite resistance to all 3 wires on the first coil. You will have to test to see what the polarity of the motor is, however.

Useful explanation of 6-wire setup from National Instruments:

If you have six coil wires, then each phase has a center tap wire:

• The center tap wire should have half the internal resistance of the full phase. The easiest option is to use a multimeter to find the two pairs of wires that have the maximum resistance.

• Connect each phase to the amplifier, and ignore the polarity (+ / -) for now. You have a 50 percent chance of guessing right. The motor should rotate, and if it is in the opposite direction, then switch either phase A and A- or B and B- (effectively reversing directions).