I think it should be enough if u receive only the nxt coins. "IF" u receive them then u can give the positive to the guy who has to send BTC. After the nxt guy receive the BTC u release the nxt to the btc guy.
Or u could receive the BTC and tell the YES to the second guy who has to send the nxt. Everyone can check if he sent those nxt at ntx blockchan explorer and check the situation.
Less work for you.
My two cents
Raúl