TM to SB Mewtwo for T3 raids?

It's a hard decision since you only have 2 TMs and might get stuck with a useless move. I can lay out your chances:
55.5% you get SB.
22.2% you end up with psychic-2 TMs.
11.1% you're stuck with FB.
11.1% You're stuck with HB.
In any case you end up without SB, you can TM later back to psychic when you get more TMs, and 55% is not bad. I would advise you to try for SB, it's very close to psychic even against Machamp and it's superior in any other case.

Yes - CTM for Shadow Ball. It's a great move for Mewtwo, and well worth having regardless.

It also means that you will have a much higher ability to do small-group Mewtwo raids over the next month, if those opportunities present themselves. (like, say, you're in a different area for a time and come across a Mewtwo raid but only 2 other players are currently able to do the raid) I would imagine that Mewtwo will motivate players into doing raids more often than they currently are, so it's possible you might be able to do more T4-T5 raids than normal - and having Shadow Ball might make the difference between being able to do the raid with the 3 people who can do the raid and having to abandon the raid because you need 4+.

CTM's can technically be dropped from any raid, so you WILL get more in the future.

It's a risk, though - many of us have horror stories of using 4+ CTM's trying to get a specific move (it took me about 7 on a Machamp to get Dynamic Punch...), so you might end up using the two CTM's and ending up with Psychic anyway. Personally, I would take the risk - I HAVE taken that risk. I used my last CTM's to change 2 of my Mewtwo's from Focus Blast to Shadow Ball, and now I'm out of CTM's. But I can do 2+ Level 5 Raids per day if I really wanted to, so I will get those back pretty quick - I'm in a much different situation than you.

The risk could be worth it, if you also think about the fact that Mewtwo could get a better Psychic move after he’s released. If you then need him to be double psychic then you can TM him back to Psychic or a possible new move. Or keep it Shadow Ball since it’s still a great move even neutral

I would at least try it. Best case scenario you get Shadow Ball, worst case you get stuck with Hyper Beam which you can TM off later. Middle ground you get either Psychic or Focus Blast, both of which have their uses.

As a solo player your Con/Psychic Mewtwo is pretty useful & I wouldn't risk changing it with just 2 CTM's. You may not even get Psychic back & be stuck with 2 moves not much good to you on tier 3's. CTM's for solo players ARE a rare commodity, players who do many tier 4's & 5's will be swimming in them. Most of my CTM's have come raiding Legendaries, had very few drops from soloable raids so I think it's bad advice for the op to use his up on Mewtwo. Yes Shadow Ball has its uses but soloing tier 3's is rarely one of them.

You can always TM back to Psychic, but not Shadow Ball.

Further, SB Mewtwo will be very useful against Mewtwo (for a month) and Deoxys (future EX raids). You can't get it after 1 PM PST on Thursday, unless they change the moves up again. But for now, it's out of the rotation.

Further, it's possible/likely that Mewtwo will replace SB and HB with another Psychic move that has a good chance of being better than Psychic. Future Sight, Psyshock, or Psystrike will all outperform Psychic, so losing it is no biggie, and, over the next month, if you get one of those rare opportunities to do a group Mewtwo raid, you can probably replace the Psychic Mewtwo with one comparably good or better, so even if you're 'stuck' with Hyper Beam or Focus Blast, you'll still have a good pokemon.

Finally, I'd try the first CTM soon, and then wait until tomorrow to do the second (as long as you beat the deadline, that is). Splitting them up over days has, in the past, gotten better results than using one after another until you get the one move you want. Could just be RNG variances, but could be a real thing, too.