My guess is you will be able to log into both just fine as always. But to be sure...just try it
Reasoning: the interop is only to be able to communicate between the two IM clients. You can even add your yahoo addy to WLM and talk to yourself...if all's good it should differentiate wether you are on yahoo or WLM incase you got your own WLM addy on WLM too (in this scenario there might be a problem/bug/limitation but i'm sure they'd be interested in that one).
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