I think it is always better to invest in a single powerful gpu card that is also on-budget in the majority of games.
Dual-graphics and CrossfireX are good technologies, as an alternative, but only when they actually work as intended. They usually don't. Not only because AMD's fault, but mainly because the devs must optimize each game accordingly. And nowadays they don't for soooooo many reasons.
Too much frustration that you should avoid, in my personal opinion.
So. go for a single gpu that delivers, more or less, the same performance in fps, with the added bonus that you will know it works 100% all (most of) the time.