I'm an MSTP alum and at least how it was at my institution was that one of the days was more casual, like brunch at the program director's and stuff. All the big interviews (for med school and the MSTP were on one day). So for the brunch/tour/hang out day, I would wear something like slacks and a blouse/sweater etc. On the formal interview day, would wear a suit.
They should make this clear to you, and if they don't, email the program coordinator to find out. You shouldn't need to bring 2 suits under any circumstance.