In my opinion, this is fine.
Blaziken is a Physical Sweeper and Magmortar is an excellent Special Sweeper. Stealth Rocks can hurt Magmortar as he is a pure Fire type, but nor Blaziken, as he is a Fighting / Fire type. Stealth Rocks will only do neatral damage. However, I would strongly suggest that you also have a Rain Dance Team counter, and one with a weakness to Grass or Ice. The synergy between to FIre type, who are weak against Water, Rock, and Ground type, and a Pokemon (who can benefit from Rain or Water type moves in general), will be excellent. You could form a Core of three Pokemon.
So, that's what I suggest. Yes, it is okay to have both Blaziken and Magmortar.