Remember WiFi but don't autoconnect on iPhone?
Forgive me if I'm missing the point of your question, but if you've connected to both SSIDs and your iPhone remembers them (and their passwords), then why wouldn't you just go to Settings > Wi-Fi to manually choose and connect to the network you want when you need to?

My point is, according to your question, you'd still like to be able to easily connect to that one when I need to (not that often), and wouldn't this approach achieve that? It's easy to do and since it's not something you need to do often, it's not cumbersome. Your iPhone will remember the password and once you've manually selected a network it'll remain connected to it while the signal is strong enough.