Would 2 Green Terrors Live happily in a 42g tank? They would be about 2" each.
If not would 1?

Unfortunately not. I'm not exactly sure how big they reach as adults, even if they only reached 8 inches, a 42 gallon tank would not be big enough for both of them

Would it be ok for one?

I don't think I would be putting one in anything smaller than a 55 gallon. However, we have some members who do keep them so they will give you the benefit of their experience

males gt can grow to almost a foot in lenght. the females slightly smaller.
unfortunetly a 42 gallon tank is a little on the small side for a mature one. however they do grow rather slowly when compared to other american cichlids so you could probably get away with it for a while but an upgrade will be necessary at some point.

You do need a bigger tank for 2 gt's, go for a 100g

Make sure you get a male and a female, 2 males will fight