No, you definitely want them both in a single zone 172.11.65.in-addr.arpa. My question is whether your ISP is really going to delegate authority for the whole zone (254 addresses) to you so that you can serve reverse DNS for two of the addresses in it, or do you give your zone file to the guys who have the authority, and they merge it with theirs (in which case you need to know what they need, not what BIND eventually will need).