You need to be using the guild training optional rules to regain casting levels (spell slot and casting advancement) merely to gain near parity with single-classed spellcasters for any Mystic Theurge build to be worth attempting.
Or be the only divine and/or arcane caster in your party, while accepting that you won't be terribly effective. It would make a decent GMPC, if no-one wanted to play a spellcaster at all.
I've found that one prepared caster class and one spontaneous caster class is perfectly viable. The book keeping required for 2 prepared casting classes can get really overwhelming. While you'll be less versatile, you'll by no means be underpowered.
I know several GM's of home games who ignore the SLA nerf, and if your GM is one of them, then this is indeed a good combo. Otherwise, Tiger Lily is correct - Mystic Theurges everywhere are better off warming the bench until another FAQ (or maybe even Pathfinder Unchained II, redoing Prestige Classes??) let's them back in the game. Just to be clear, I haven't heard anything about either of those, just wishful thinking...
^I would have reversed your 3rd sentence -- a Mystic Theurge is very versatile once it gets going, but really underpowered without the pre-nerf SLA FAQ, until you get to really epic levels.
An alternative to a GM just ignoring the SLA FAQ nerf would be to make a specific fix to Mystic Theurge (and to arcane Strike and Eldritch Knight, for those so inclined).