I am by no means an expert on crossfire, but I can tell you this: "it depends"
Crossfire means you leverage the gpu power of 2 gpus working together. This does not mean they power different monitors.
Problem is that your game needs to be optimized for it. And most games arent really. On top of that you get an insane powerdraw, fitting problems and general hiccups with getting it to work at all. And yes ideally you want to have 2 nearly identical cards.
If you can get a card really cheap and your game supports it...maybe.
Btw how much vram does your card have?