Hi, what counts the most when you train for 2 race days back to back?, lets say 2x250 TSS 0.85 IF
from my experience I would recommend if you incrementally work up to your hard killer back to back days over time. Your body will adapt and you will be able to maximise your effort an minimise your discomfort and or risk of injury.
Remember that not all TSS is created equal.
An hour at around FTP, so a TSS of 100, would be brutal for me. Four or even more hours at 70% of FTP, i.e. still in Z2 for both power & HR, so TSS of 240 upwards would have negligible effect. I’m assuming decent refuelling and hydrating for both scenarios.
I find the TSS calculations to fall apart once you get to long rides, by long I mean twelve hours or more.
You don’t say what your “big days” involve: are they a weekend of multiple crit style races; a two day grand fondo; a point to point charity ride? If you only ride crits and you are aiming at long days then time in the saddle will be important for comfort, etc.
Thanks for answer, i did specify a bit more what type of biking im thinking of in the first post.
Well no, you just stated TSS of 250 and IF of 0.85. The numbers in isolation don’t mean anything. Have a look in the workout library - that’s something like three Brasted or three Mount Goode (just randomly selected), completely different rides.
Edit: Look at Disaster (yeah really), IF of 0.79 and TSS of 260 another completely different ride. /End of edit
More info and better info will lead to a better answer. 
not sure of your full training history, but start with back to back rides that won’t crater you.
Maybe 4h sat and 4h sunday, self selected pace.
See how the metrics shake out.
Eventually I like option two where you ride longer on Sat and then same intensity on Sun as the upcoming rides.
These are broad brushstrokes though without monitoring all your history as an athlete.
Good luck!