is it possible to combine 2 transistors to double the current output
As to your first question, feel free to furnish a schematic.
You could add a capacitor in parallel to the battery near the motors.
can measure their current draw when you're running them directly from the battery?
For a power supply the transistors are not in saturation.I also have a power supply with three parallel 2N3055.The transistors didn't match very well, but I compensated that with a slightly different base resistor.If a transistor is used in saturation (as a on/off switch) a very minimal difference will have a large effect.