One of the most effective "buddy teams" is a Striker and a Leader. Specifically, a striker that gets her striker damage every turn instead of round on a melee basic attack, and a leader that grants melee basic attacks.
From there, we can look at your requirements: Eladrin twins at level 1, with high mobility and stealth.
Twin 1, therefore, is a Thief. The thief "build" has been mostly solved, and I would borrow heavily from builds featured here. Alternatively, an eladrin can make an excellent cunning-sneak rogue, if you want a little more complexity in your build.
Twin 2, must be one of the trifecta: Warlord, Ardent, Shaman. Warlords, unless they're lazy (and boring) really would like strength, but you could play an exellent Insightful Warlord. If you go this route, make sure to allow the warlord to roll her granted attacks. Otherwise there will be very very little rolling for her. Ardent is also an option, and my favourite class. Unfortunately, the ardent is slightly suboptimal due to the wording on "Ire Strike" which requires an adjacent ally, and therefore is more difficult to present the ardent as a flanking buddy versus "Commander's Strike" which does just that. On the other hand, it's more fun for "back to back" fighting. Shaman is good, but out of the running due to your requirements for melee shenanigans.
With more details, we'd be happy to make specific builds for the twins in chat.