Shims work fine. We have 2 on our bike: One for my standard dia. Deda 215 bars to fit the Captain's Thomson OS stem, and one on the Captain's seatpost to Stoker stem interface (That one is a seatpost shim that we cut down in height to just fit inside the stem's clamping area).
We use the Oval Concepts R700 for our stoker stem since it fits her 26mm dia. bars and it is available in 140mm and 150mm lengths with barely any rise/drop.
http://aebike.com/product-list/oval-...49-m2684-qc30/