[TYPO3-mvc] query for match two users of same usergroup

[Domi]

Hello,

I have a little message system, where in my MailEntry I store in:

fromUser Tx_Extbase_Domain_Model_FrontendUser
toUser Tx_Extbase_Domain_Model_FrontendUser

Now I want to check, which fromUser and toUser has the same usergroup.

I tried with:
$query->equals('fromUser.usergroup.uid', 'toUser.usergroup.uid');

which is for sure wrong.

Is it possible to get this however work with extbase query statement? 
Can somebody give me a hint which would be the best way for it? I have 
problems with solving the comma separated list of usergroups...

Thanks!

Dominic