You can't.

I press F10 and it lets me choose if I'd rather carry two pistols or two primary weapons.

You can,but to do that,you will need to have to either re-create the map,or create your own map.

First,you need to get and know how to use Valve Hammer Editor. Next,when you have done creating your map layout and miscellaneous effects to the map,you should add these entities :

multi-manger, gameplayerequip and playerweaponstrip

These entities will enable you to have your desired weapons on hand when playing the map. So,you want to have M4 and AK together? Just strip all the weapons,and equip back M4 and AK only.

To read up more about the entities,refer to this tutorial.

This only works in multiplayer games, on the internet:

Press ALT+F4 at the purchase site, it will come up with a new buy menu. In this buy menu, it allows you to choose a secondary weapon replacement, for example, equipping an AK47 instead of a pistol, so you have both an M4 and an AK. Hope this helps ;)

Also, if that doesn't work, try pressing ALT+TAB, that should work too.

Lol hitman 407 alt+f4 does someyhing different lol. but nice try :p

If you read the posts in this thread, well that is how dumb the most people are on fpsb. The most people does not help, they try to make it worse. Shame on you!