Is it polite to ask for your potential supervisors to help get the scholarship?
It might be a bit risky and it is impossible to know what the reaction would be. However, if you actually need the scholarship in order to be able to accept any bid, then it would be appropriate for the supervisors to be told that. 

If they really, really, want to accept you then it would be to your advantage that they know that you have financial constraints. However, if they are a bit ambivalent toward your acceptance, then it might be easier for them to invite someone else. 

But, since you know one of them, and think he would support you, you could ask him to make your needs known to the other, rather than doing it directly yourself. 

And it does no-one any good for you to hide the fact of your need.