Versioning is always had in BizTalk. In fact, I’ve never been in a situation that we had to version.
I can’t think of a good way off the top of my head to accomplish your solution. I think it’ll be hard to have a single server to support both A1 and A2 without making code changes on your client side.
If you wanted your service to send to A1 and then switch to A2, I think that would be more doable by modifying the way the web service published the message to the message box. See my sample on Submitting Xml Data via SOAP.
Hope this helps.