[GeeTwo]

[As always, assuming the rules for this year are consistent with recent years] - Bottom line - yes, you can make do with one set of electronics, and transfer it from your "practice" robot to your "competition" robot on the first day of a three-day event, or during your last "unbag window" if you're doing two-day events.
However, doing so would greatly reduce the value of the "withholding allowance". Any item (whether control system, manipulator, sensor, or otherwise) transferred onto the competition robot after stop build day must either be broken down to individual COTS components, or counted against an event's withholding allowance. Even before we built a "second robot", we found it highly useful to have a "second control system" so we could bag one wired into the competition robot and work with the other between stop build and competition.

Edit: having two robots and one control system will support drive practice, but will probably not support much improvement of your drive system or manipulators after stop build, unless you're uncommonly fast at pit builds.

FWIW, we have always (yes, back to rookie year) used our withholding allowance for a manipulator (or two). The only time we withheld our control system was rookie year, when we did transfer it part-by-part to the competition robot. I am still amazed that we managed to pass inspection before the first round of competition.