I have never used or seen the interlock kit that Ron posted but they also make an interlock that mounts over two 2-pole QO breakers mounted next to each other in a loadcenter. It also secures both breakers as required for back feed applications.
Look at the PK4DTIM4** kits in the middle of the chart here:
http://ecatalog.squared.com/catalog/173/html/sections/01/pdf/17301008.pdf Curt
