Using both Apple TV & Airport Express in sync

I thought that when I use Airplay to stream my iTunes library from my iMac, I would be able to hear the same output from both my Apple TV and my Express. It's not happening.
I am using the remote app on my iphone.
On the iphone I see 2 icons; one for the Apple TV and one for my iTunes library.
If I select the iTunes library, I get output from the Express.
If I select Apple TV I get output from the Apple TV.
But never from both at the same time.
When I select my iTunes library, I can see that both units are available as speakers, but it doesn't work.
I have checked that both units are on the same network.
Anyone have any ideas what to check next?
Thanks

All I can tell you is that it does work for me.

If the AE and TV are using the same receiver then most likely you can't listen to two inputs at the same time.

I use the Apple Remote on my iPhone. When selecting the icon for outputs I can select the AE and TV as well as my iMac.

and guess what - now it works for me!!!!

I hope it remains stable, but it does seem to be a bit hit and miss.
I have been reading on the web about how it is best to use wired connectivity for streaming, as there can be too many factors which can make a wireless set up fail.