I have two Roku soundbridge devices. I would like to be able to listen to the same audio on both devices simultaneously.

I know I could fairly easy send the same play commands to one unit that are currently playing on another unit, but there will be all kinds of network lag that will cause problems.

Basically I would like to have a new source option on one soundbridge to be 'play what is playing on soundbridge X'.

So, is it possible to have the soundbridge duplex its incoming ip packets to another soundbridge?

Try some software called CIDERO. Ive tried it & it works fine.

Yes - Cidero works, but it may not always get the synchronisation exactly right. But then nothing will unless it is designed to from the outset.