N5269 - I am going through a similar thing right now, although mine is manual controlled instead of a RIMS (for now).
First you are going to need at least a 30Amp breaker. I might suggest that you check out the CH Spa kit (~$70) at Home Depot to use as your on/off switch. Includes that GFCI protection.
Here's what has worked for me. I have the 240 line coming into a junction box. I used some "grounding" strips (found near the circuit breakers at a box store) to spit off the power. Make sure there is no way in hell that these can touch any other line- mine are separated by a panel, secured in place, and covered with electrical tape. THis will split your output into the two heater sides you need.
Slimer - to do this, you have to come up with some additional electronics to do the switching. Something along the lines of 2 x 25 amp SSR's and a couple of 555 timers or an Arduino board would do the job. I might could help out on that. PM if interested.
DISCLAIMER: Electric + Water can = DEATH. If you aren't sure, contract with an electrician to be safe. It could be the best money you ever spent.