Sorry, but I don't. Just a guess, but here goes-
It seems there are 2 ways it might be plumbed now, 2 in series or 2 in parallel. If in series (my guess) just add a line as a bypass between 1 & 2, then put a 3 way valve where you split the two, after you bypass the 2, T back into the line after the second membrane, have a std valve just before the T. Both concentrate and permeate will need this.
If in parallel, just add a valve in each line, concentrate and permeate to exclude the second membrane when desired.
Regardless of how you run it, I think it advisable to was it as a 2 tower unit.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.