Some ships were used by both factions, such as the dreadnaught.

so, my question on this is, will we ever see ships taht will be used in both factions, from the same expansion?

I very much hope so. I think it's thematic - the Rebels use much of the same stuff as the Empire and always has. And both use stuff that was once Separatist.

EDIT: Just look at X-Wing vs TIE Fighter. We've got a Nebulon B and they've got one too. So does that guy over there. Erryone's got a Nebulon!

Its the Oprahlon - B!