That game recommends an R9 380, which your 280X is faster than. Also, the game developer has not enabled multiple GPU support, so there can be no Crossfire profile until there is. From the Planet Coaster Forum Moderator:
The other thing to consider is that the work required to support this type of configuration isn't free. DirectX 11 doesn't do multiple adapters very well (as far as I understand it) so it can be a fiddly job getting it right. The number of systems configured in this way is relatively low, so whether the investment is worth the return will ultimately determine if it's worth doing. Perhaps when DirectX 12 is more widely supported, and assuming it's multi-adapter support is as good as promised, we might see games able to take advantage of the capabilities of these multi-GPU systems without the nasty hacks and driver patches that are currently required.
Just disable crossfire for this game in Radeon settings>Games>Planer Coaster game profile(if it not exist - just create it)
FreeSync may have negative effect too in your case.