Yes, you would save 1 tractor (for what, I'm not sure) and fuel/driver.. but what I'm looking at is acres per hour.  1 machine sitting because of rain is the same acres per hour as 2 smaller machines sitting because of rain.

The real answer is how many acres per hour can be acheived when both machines are running in the field sizes you are talking about ?? 

Same can be said for 2 smaller combines vs: 1 large machine. I did look at a couple of posts and 2 mph seems to be awfully slow.  The machine is not set up correctly, or there is an unseen/unknown problem.  DO you have a shop ?   Hopefully a shop and time to really tear apart a combine and really fix the problems.

One advantage to 2 older smaller combines is know "problem parts" can be stocked and only 1 is needed.  1 newer combine still stock 1 part for "problem parts" but only 1 machine.  What is the actual acres per hour on a properly set up 7720 vs: newer combine?? What is the actual acres per hour on 2 7720's vs: 1 newer combine?  What is the actual cost per acre for maintaince on the 7720 ??   (lots of expensive repairs does not indicate very much) 

Honestly,, what is the life expectancy of the 7720 for getting parts etc??  What is the life expectancy of a newer combine for getting parts etc??   What is the real ROI for each machine taking into account the actual acres per hour possible with each?  

Same weather issues apply on harvesting too. 1 machine sitting because of rain is doing the same acres per hour as 2 machines sitting.

Note: I purposely avoided most all the posts but did read about the low 2 mph as a limiting factor, so my answer would be strickly looking at the real answer which is dollars per acre return.   The real answer of course is simply changing the weather,, but that option isn't.