How do you combine items?

[Edegen]

I see when I click an item, another item has an up arrow on top of it, how do I combine the two?

[N.I.T]

you only combine guns , by using duct tape ..
if you see an item has a up arrow on top of it that means 2 items have synergy, you dont need to combine them

[Smakit]

You don't generally combine items, they just auotmatically get a bonus while you own both (that is, if you drop / sell one, you'll lose the bonus). For weapons it tends to change the look of the weapon slightly and will do different things than before (have multiple ammo types, fire on reload, bigger blast, etc.etc.)

If I'm understand you right, the up arrow is just showing a synergy comes from that item. I mentioned in another post that the only one I ran into different so far is MegaHand (looks like Mega Buster from Mega Man) which you may have to hold your reload button while it is equipped to switch ammo types.

Duct Tape is the active item that you can actually use and need to select two weapons to put together. Also there may be item synergies (passives) that won't change a particular weapon, so it just works in the background.

Edit: I took too long typing. What Cl4p-TP said ;)