"Making Mono" for checking Azimuth

I have a Hifi News test record for checking azimuth, but it requires minimization of a signal when listened in mono. My preamp (like many others) doesn't have a mono switch. Can I just take the output from my phono preamp, run it into a Y adaptor, and feed the output of the Y adaptor into one channel on my preamp, and just listen to that channel to minimize the signal? And further, could I reduce 2:1 with a Y adaptor and then expand 1:2 with another Y adaptor, thus creating 2 channel mono?

Or - is there another way to check azimuth that I should be considering?

Thanks, Peter