I've been known to go out with my high cycle MP5 with a snail mag on a sling, my MP9 on one hip, my MP5K on the other, and a fully auto Glock 18 in a holster. It's bloody heavy, slows you down and makes it a nightmare when reloading, remembering not to put the GBB MP5 mags in the MP5 AEG and vice versa was the main issue!
Generally one weapon will be plenty, unless your main weapon is a little cumbersome and you want a pistol of some kind for tighter situations. I only play CQB so all of my guns are fairly compact.