Gam3r01 :
Nope they cant. They have the be the same cards 2x 270 or 2x 280s or one with the same board. So a 280x and a 7970. That is the only exception.
not true, look at this chart:
you can crossfire any chips that use the same architecture/die size, essentially just look at their codenames, if they are the same or are of the same family they can be crossfired.
For example, the HD 7870 XT, HD 7950, HD 7970, and R9 280x all use a Tahiti processing unit, so they can all be crossfired with each other.
the HD 7870 (regular and GHz), HD 7850, and the R9 270x all use the Pitcairn processing unit and can all be crossfired with one another
the R9 290 and the R9 290x can be crossfired because they both use Hawaii chips
but OP, in your case, no, the R9 270x and R9 280x are of different families and cannot be crossfired with one another