I think you can only have one monthly device payment active at a time. So quite certain you can pay off the remainder of the payments required for the S6 edge as a bulk amount, and then add a new monthly device payment arrangement for the S7 edge.
@mrhandyhaydn - @Dylan is correct - You would need to pay off the rest of your current handset in bulk (i.e your next invoice would have the remained of the RRP added to it) before you could add on another interest free device.
Your other option is to apply for a 2nd connection (new number/plan etc) and add on interest free. You would be liable for both connections however.