How to approach potential supervisors who have already expressed an interest in supervising me after submitting my grad application?
I don't think the rules require that you decline the other professor formally. I probably wouldn't do that. Reaching out to both could get tricky as you will need to choose at some point anyway. 

For the introductory letter to the one you prefer, introduce yourself and say something about your own interests and background (very short). You don't need to praise the professor. Saying that you have read a bit of their work and if some particular part of it is interesting to you then say which. 

I assume that you will get an invitation to meet since he/she has already expressed an interest. You don't need to praise yourself either. The prof will have an idea that you fit the requirements. It isn't a selling job, but an introduction and statement of interest. 

It is too early to make decisions. Wait until you meet to jointly figure out how you would fit in. But that would come after acceptance.