From the Scepter Tower of Spellgard, the official module set in Forgotten Realms, an adventure for level 2-4, there is a double item. In room T4, there is an item called scepter of arcane winter which functions as both a wand of shield +2 and a wand of icy rays +2. I thought there was also a Thundering Ice mace in there too, but I can't seem to find it, so that may have been something I made up as the GM.
Ok, so it's just a wand, but there's no wands with 2 spells in the main book either. I basically go by the theory that there can be items with 2 powers in them, but they are immensely rare. As for level, I go by 1.5x the level those items would be separate. So, say you have two powers you want to put into one sword, and individually those two powers would each make a level 4 magic item, a sword with both of them is a level 6 item.
Fighter: "I can kill a guy in one turn."
Cleric: "I can kill a guy in half a turn."
Wizard: "I can kill a guy before my turn."
Bard: "I can get three idiots to kill guys for me."