Molybdenum/Tungsten alloy armor piercing round
Probably not. Molybdenum and Tungsten are both hardening alloys to steel, they perform the same function, though differently. I suspect a Mo - W alloy would have the fragility of both molybdenum and tungsten while not hardening each other especially much nor increasing density.

The answer is: Sure, it would be hard, heavy and likely armor piercing, but very inefficient use of both Mo and W. Just use W alone, or alloy it with elements that are lighter, but melt within reasonable ranges. A quick glance at wikipedia confirms that Fe, Ni and Co are usual alloying elements. Tungsten carbide is also used.

Creating this Mo-W alloy would also be very difficult, usually you solvate the high mp material in the liquid low mp material but these both do not melt within reasonable limits so you would have to use some seriously high temperature equipment, we are talking beyond bright white heat - or rely on very, very slow processes (diffusion - and these are large atoms...).