please consider the illustrated T. i can untwist the left (9-o'clock) because it is sawed off further to the left (not in pic). but how the heck do you detach the 12- and 3-o'clocks because turning either in the direction that will untwist it from this joint will be tightening it further on the other end ?
i hope i do not have to saw it.
i have always asked myself this question before i got the house and now it has escalated into an actual conundrum.
I would saw through the "12:oclock" piece. Then do whatever work you need to do. Put it back together and replace the sawn 12:o clock piece with 2 nipples and a union. (it looks like there is a coupling a foot away or so). You should be able to have the nipples custom cut to length so they fit right if necessary.