Rigging - how do you mirror limbs across a spine?

I remember seeing a topic like this but for the life of me I cant find it again... most of you can relate, Im sure. :)

Im (re)rigging an animal There is a spine that runs down the line of symmetry, and then legs come off of the spine chain. I want to mirror clones of its limbs from one side of the spine to the other, but I want to keep them attatched to the spine at the same time... is there a way to do this? Or if I cant do that, how can i attatch multiple bone chains so they become a single chain (as opposed to multiple chains that are linked but still physically seperate)?

thanks, as always