Do you need to receive them at the same time, say in a convoy pattern? Or do you just need to receive either of two formats and apply the same logic to them?
If it is a convoy you are after maybe look at the following link. it is from the 2004 days but still applies to what 2006/2009 would offer.
http://msdn.microsoft.com/en-us/library/ms942189(BTS.10).aspx
Or if you need to receive one of two message formats I believe that this link can help.
http://www.codeproject.com/KB/biztalk/UntypedBiztalkMessages.aspx