"In Hand" gun will disappear/despawn if you use any item in your inventory. Eating/drinking, flashlight, compass, etc... If you want to carry 2 guns the safest way is to set the 'in hand' weapon on the ground before using any inventory items.

It is possible to carry 3 primary firearms at a time due to 3c and log out/in will despawn both firearms only the inventory firearm that is attached to your character will remain.

When using items with holding a 2nd gun in your had the character will always drop it in the ground. It can get lost (dropped into geometry, etc) especially if you're backed into a corner or being too close to a wall (for climbing a ladder), but it will always be dropped.

The bigger issue here is that when you have 2 rifles and you log out/in it's possible to sprint/turbo because there's nothing in your hands.