For Nvidia It's called SLI, not crossfire which is a term for multiple AMD graphics cards in a single configuration. And, no, you can't SLI with 2 different graphics cards. You need two of the same models, however you could SLI an MSI GTX 970 with an Inno3D GTX 970 for example but it would look a bit weird in your system if you have a windowed side panel so two of the same is usually recommended.
But this also depends if your motherboard supports SLI at all and if you have atleast got a 750 watts power supply. If you don't have either of the options, you already can't.