yeah, "Amateur", that part always amazes me, too, when i watch multi-dog household videos! HOw the other dog(s) sit patiently waiting their turn, even when treats are being fed! wow. THAT in itself, seems a great trick to ME! lol.
. If you really want 2nd dog to shake his head at same time---- Here is how to teach the head shaking, you could teach dog to
go side to side,
or up and down: http://www.dogtrickacademy.com/members/forums/threads/shake-head-no.2244/
that "shake your head" was one of the hardest tricks i ever taught Buddy though, lol. But, i also made many mistakes in trying to teach that cue, another person might not find it so hard to teach as *i* did.
Of all Buddy's tricks and cues, very few i label as "hard to teach" and 'shake head' would be one of them!
but, i hope you DO buy a sturdy toy drum for Eliot! Or trim up some object to look like a drum, it'd make this trick even cuter.