2nd charge move

Shadow ball/Focus Blast is use full in raid and gyms. But Focus blast is not ideal in PVP since its a single bar move.

I'm also thinking to add a second move to my SB Mewtwo, but I probably power my another Mewtwo (better iv one) as SB is already a strong move for generalist. I can spend the dust and candies to power up another Mewtwo say with IB and TB combo to have yet another useful Mewtwo generalist.

As a proud owner of one I can absolutely say that yes, it is worth it. I have enough SB Mewtwo to have each one of the current charge attacks as a secondary move on a legacy one, and will definitely do that sometime in the near future.

I gave focus blast as the second attack to my best shadow ball Mewtwo, and I really like it. It's very powerful and has a lot of versatility, so I consider it one of my best investments.

In PvP, with the quick energy gain from psycho cut, focus blast doesn't take all that long to charge up, and the combination of FB and SB gives it a lot of powerful and very relevant coverage.

It has also become one of my new go-to Mons for attacking gyms next to MM Metagross. It has better survivability compared to Machamp, and charges up moves very quickly. I'll use focus blast to bring down anything weak to fighting, and shadow ball for everything else.

In raids, you're likely going to primarily use shadow ball anyways, but the addition of focus blast does give it a wider range of coverage.

This may not be the most "valuable" investment of resources, but I would do, and I plan to with my own SB Mewtwo. In PvP, Mewtwo with Psycho Cut, Shadow Ball and Focus Blast would be an incredibly powerful sweeper, something you keep in the third slot to bring out once all of their shields are gone. Additionally, Focus Blast is nice in Gym Attacking.

IMO a second charged move on Mewtwo only makes sense for PvP.

Also, mind that adding a second move of your choice will require a large number of TMs (unless you are really lucky).