I did LP and Tremblant back to back 2013 and in 2014 I did Whistler and Tremblant back to back. Both cases they were 3 weeks apart. I anticipated that my swim and bike would be just as good in both races and they were. I planned for my run to be around 10-15 min slower in Tremblant and it was in both cases. In between races, I swam mainly week 1 with some light bike commutes. Started jogging a bit at 7 days out (like 15-20 min). If I recall correctly, second week I kept up those short jogs (10 min to pool, swim 30-40 min 10 min jog back), and bike commuted but made one of the bike commute legs (only 30 min) relatively hard (TT mode). After 10 days out, all my swims were hard. 7 days before my second race, I did my standard 90K bike loop from home with my standard mid ride 20 min TT in the middle to check my power and how my legs were doing. On both years, on that TT, I hit my highest power for the year (post IM super compensation and really a ton of rest given taper for race 1 and all the rest post race 1).

As part of this, my taper for race 1 was barely 1 week. The reason being I did not want to have a 3 week taper, race and 3 week recovery. Basically I had 1 month of easy/low volume stuff with 2 IMs (the first one at the 1 week point the second one at the 4 week point). I felt with a 3 week taper for race 1, I'd have less fitness left for race 2.

I also found the Whistler-Tremblant double much harder because of the travel post Whistler (I actually had to leave Whistler day after race and fly to a customer meeting from Vancouver for a day, and then fly back the same day to catch up with my family and finish vacation and then fly home). All the bouncing around in the critical 72 hour window after Whistler really put a dent in my recovery.

I would say the most important thing is what you do in the first 72 hours after race 1 and not doing anything stupid from 3-20 days after, but once you can jacking up the intensity on swim-bike and keep volume low and just jog easy.

Oh, also on race week, no running from Tue onwards. Also I tried to do daily 30 min walks for 14 days after race 1. Keeping 3-4 hours per week of weight bearing aerobic work on the legs to keep things on track for the second marathon with zero trauma from pounding

After the second IM in three weeks you developed sciatic pain?

Not sure where you came to that conclusion, but sounds like your question is loaded. I was fine after both of those doubles for many months and raced around 15 other times in the next few years (spread between tris, running, XC skiing, winter tris). No problems during those times.