that's all being academic, since IoT will broadcast their presence to the "world", hence giving away their IPs address and therefore the subnet attached to it, hence giving routing access to any "sniffer" already present on any of the subnets.
no advantage, and it's not an opinion, it's simple facts. If it was so easy to have a secure network with 2 pools of IPs, everyone would have taken that solution long ago, and it was what was done in the late 90s, early 2000s, but now, not making any sense, IPs are the least of your problem for security.