Okay, stuff that would work well for both:
Bow of Destiny or
Wand of Reality (or both; they're decent paired)
Golden Compass,
Scuzzys Comb or
Ramtors Spellbook (I'd pair any of those with the BoD rather than the WoR to avoid too much light, although if you're going to overload on an icon type in 2-player, light's not a bad one)
Honey Potion (not as useful once both your main attackers do 13+ icons, but if you go with one 13-iconer and a 9-10-iconer, it's still got a place)
Greater Healing Scroll (until you get to 100 hit points, at which time you'll probably want to upgrade -- unfortunately, the next real non-species upgrade is the much more expensive
Bag of Healing Dust)
Downsize! (it's just always useful)
Ring of the Lost (probably moreso in 2-player than in 1-player, although it can certainly come in handy against 1-player opponents who deal a lot of dark)
Stuff that would be more useful in 1-player:
Black Frost Cannon (you can get lucky and hit with it in 2-player, but it's a percentage freezer and it's better suited to 1-player, where you can simply withdraw and heal if it doesn't freeze when you need it to)
Yooyuball Player Sling/
Yooyuball Keepers Chest Guard (physical is very common in 1-player, and blocking all of it would help you a lot. This weapon could be useful in 2-player as well, but I think of it as more of a 1-player thing).
Prickly Potion (it does only six icons, but it does those six icons using the
opponent's strength boost, so it can really help against strong 1-player opponents. Quite a few people use two at once for an even harder hit)
Stuff better suited to 2-player:
Dual-duties like
Mask of Coltzan,
Dusty Magic Broom/
Air Faerie Crown,
Hobans Hat; pure shields like
Ghostkershield and
Faerie Tabard (defence is useful in 1-player, but only to a point -- if you're going for avatars/fighting in a war or NCW event/trying to get on a high score table, you're going to be fighting opponents with a much higher strength boost than your defence boost, so your shields won't end up blocking much. In 2-player, you'll be fighting people with roughly equal stats, and defence will be much more useful)
Jittery Jipple Potion/
Steaming Skeem Potion (in 1-player, they do only 6 and 8 icons, respectively, but in 2-player -- as long as your opponent has 10+/19+ intelligence -- they do a full 10 and 11 icons, respectively)
That's all I can think of right now... I'm sure others will weigh in!
...or maybe some standard items for both ways, with a few that would be switched depending on 1p or 2p?