When to follow up with a potential supervisor after sending referral contact details I have recently contacted a professor for an advertised phd position and we had a meeting to discuss the details. After this meeting, which went reasonably well, he asked me to send two referral contacts which he will directly call. Two weeks have passed since I have sent them. 

I think it is not too long since it might take some time for him to call both of them (since he and also the referrals are quite busy all the time). However, I'm a bit unsure if I should send a reminder or follow-up e-mail soon to ask the status.

What is the appropriate waiting time for that?
It is impossible to say in general. It is a busy time for many academics. If you are friendly with your two referral contacts you might ask them first if they have been contacted. If they have not, then there is some delay and you should probably just wait a bit longer. 

If the start time of the degree program isn't soon, there is little reason to panic. But if you want/need to start soon, you may want to contact the potential advisor sooner. 

However, in some places, acceptance to a program isn't controlled by the individual faculty member but by the university. In that case, make sure that your application is proceeding normally. He may need to wait for a signal from them before he spends time on a potential student. Other places, professors directly hire into their own labs.