[EricH]

Team 330 used the kit gearboxes with one motor to prototype a drive, then converted them back to two motors per box for competition. If one motor doesn't work for you, then expect to take about an hour to 1) pull gearboxes from mountings, 2) disassemble gearboxes partway, 3) add second motor, and 4) reinstall gearboxes.
And to answer the second question, unless you really screw up there should be no major complications, other than the ones your drive may already have.