DON'T DO IT!
Leather conditioner will soften the leather. Your molded leather holsters were formed to the shape of your guns and the conditioner will 'release' the leather so that they will no longer snugly grip your pistols.
Of course if you are using cheapo 'pouch' type holsters, it won't affect the leather any, and the conditoner shouldn't harm the guns. Conditioner on the gun may scortch and smoke if you shoot the gun though.
Just wipe down your holsters with a wet cloth and maybe use a bit of wax.