Potential PhD supervisor mentions potential co-supervisors in email. Should I contact them?
That depends on what else your Prof. said in the email - (s)he may have mentioned that they will initiate contact : in which case wait.

Or they may have said or implied for you to initiate formal contact (as they have already chatted over a cup of tea... and want to see how you handle it..!).

If the email is bereft of suggestions then perhaps the best is to ask your Prof. if you should go ahead and contact X and Y...