What do you mean by more? If you mean like 2 then yes. But you got to crossfire or sli. Crossfire if amd and sli if nvidia.
You couldn't do it if you have 1 amd and the other nvidia card. And if you mean better, then yes offcourse
it is very dependent on which gfx cards. current gpu vendors, amd and nvidia, do not allow to combine their gfx cards. however, they allow select models of their own gfx cards to combine and offer higher performance through crossfirex and sli technologies. each company has it's own rules such as nvidia requires a motherboard with two pcie (2.0/3.0) x16 slots running at at least x8 speed. crossfire is more forgiving, allowing for running two compatible cards at x16 and x4 slots.