To mix and match guns easily do the following:
1. Put the all weapons cheat on and "x2" cheat on
2.go to a double weapon (where you have two of the same weapon)
3.press A as to go to the next weapon, but instead of following through just before it gets to the next weapon press Z so it goes back to the last weapon. While it is switching between guns release Z and then press Z.
4.The effect this should have is the effect of one gun in one hand and a different gun in another hand.