2nd mewtwo to power up?

I would only power up mewtwo with 15 attack, as I get an ex pass pretty much every week, so mew2 aren't rare, but the ivs make very little difference, so if you are playing casually and don't get regular mew2s then don't worry too much about iv.
There are 5 upcoming legendaries weak to shadow ball, and shadow ball mew2 will be an optimum counter to most of them, so shadow ball mewtwo is most definitely a good choice if you are after raid counters.

Focus blast on the other hand is pretty much useless and completely outclassed by machamp, which is much easier to power up as it doesn't require rare candy, so tm that one to shadow ball (or psychic of you are trying to solo machamp) as for your 11 attack mewtwo, if you want yo get a better one, getting ex passes is easy enough, all you need to do is do 1 raid a week at your local ex gym(s) and you are pretty much guaranteed to get a pass, I don't know how feasible that is, but niantic have completely devalued mew2 (but that's a discussion for a different thread)

A majority of the Gen 4 legendaries are weak to SB Mewtwo.

How often are you able to participate in EX raids? Do you have the TMs to change your FB Mewtwo to SB and back whenever you need? If you think you'll have more chances at a Mewtwo then I'd hold off on the one you have. Unfortunately the Gen 4 Pokemon aren't in the breakpoint calculator so I couldn't tell you if 11a will miss any. You'd preferably want Psycho Cut in these matchups and with PC being so low power it's hard to say if there will be any relevant breakpoints 11a will miss but even 12a will hit.

Wait until...
A. A raid boss comes where another mewtwo would be useful.
B. Mewtwo is no longer availible in raids.