Negative input, negative output switching regulator?
I've been using Recom converters before, really good devices. I don't think you can wire this one to draw power from negative rail, though.

So, if you want to use this particular device you'd have to draw from positive line only.

However if you open to try other Recom products, you can get any isolated dual 15V output part that accepts 56V, like RS3-4815DZ-H3, and connect input between the rails. So, you'd only need one converter and no other parts.