Optional BizTalk messages

I've been scratching my head over how to get two messages, one of which is optional, into BizTalk and linked up.

I tried a sequential convoy, with the second receive shape waiting for the second file, but as the message type differs, this isn't allowed.

So I tried a parallel convoy, with the second receive set in a listen shape with a timeout. This too is not allowed (cannot have a timeout on an activating receive shape).

The solution is to do the second one, but instead of timing out with a listen shape, it's to use a long-running scope with a timeout set on the scope. This seems to work fine, if your optional message doesn't appear within the set period, it throws an exception (which is duly discarded) and everything continues on it's merry way.