Here’s an untitled tool designed to help you find synergistic multiclassing in 4th edition. It takes into account which weapons/implements are used by the classes and what stats are important to them. It rates them on a scale from 0 (no synergy at all) to 5 (ideal match, meaning the classes share a primary and secondary ability and an implement/weapon).
Worth noting that for the weapons I didn’t actually list all of them, just the ones which could be used as implements. It’s all good, though, there are enough of them and enough variety that any weapon-using classes will show up as matching in that respect.