For two communicators of different makes to pair up, they must be in universal intercom mode. I think that I read that older communicators did not have the universal intercom function. I know nothing of Cardo so I don't know how old a Q2 is, but you might want to check to see if it has this function. I know for sure that the Sena 20S does.
As for the Sena, to pair it with a non Sena brand communicator, you must be in the universal intercom pairing mode, not the regular intercom pairing mode. I would guess that the Cardo has s similar set up and may need to be in a universal intercom mode as well.
I have used the 20S with another cheap no-name brand communicator and it paired up fine.
Just a note;
While you are in universal intercom mode, the distance at which the two communicators will work is less than it is between two Sena communicators.