I think the Weapons Plaques take Staffs. That's almost horizontal.
The single-weapon plaques are definitely horizontal. I just activated one in Windstad manor and placed a sword on it and it is horizontal.
I haven't tried it, but he shield plaques allow you to position a left-handed weapon and a right handed weapon on either side of a central shield. Maybe you could do that with two staves?
The most impressive armory I've seen is in Severin Manor over in Solsteim. You have to do the quest Served Cold to get the house but once you do, there's an awesome room with an arcane enchanter and, like, two weapons plaques on the wall.
This does work. I have two staves that have the single crystal at the end (kind of looks like a spear!) and have them mounted behind Spellbreaker.
Those criss-cross plaques don't work though for items that can't be a mirror image of themself if you flip it over. Example: you'll have one war axe facing up, the other down, but both in the same circular direction (I think counter clockwise). Also scimitars don't work either. Not sure about battleaxes, but typically only the double sided ones work...things like the Orcish or the Daedric won't. And warhammers? Same deal.
I wish there were more single plaques in the Hearthfire houses.
Hjerim Hall or whatever has that great secret room...enchanter, alchemy table, plus several single plaques.
It's only the single weapon red plaques that will hold bows. They won't go into vertical weapon racks for the racks that have two weapons and a shield spot.
I have plenty of bows in the normal weapon racks. It is only the shield plaques you cannot put bows on.
Really? Every time I tried, it told me bows won't fit on those racks. Only the single-weapon plaques will accept them. I may have to try it again some time.