Doesn't the new M4 iPad Pro support 240fps? This is from the specs page: Slo‑mo video support for 1080p at 120 fps or 240 fps.
Having two apps isn't going to work. They have to provide useful functionality on their own.
If you combine the features into one app, and you only ever want the camera part of the app to run on an iPhone, then:
If running on an iPhone allow user to select Controller or Camera. This will allow a user with two iPhones to use one as Controller and one as Camera.
If running on an iPad, only allow the selection of it as the Controller. In this case I would check which models can support 240fps and allow it to be used as Camera or Controller if that iPad can support it.
I understand how smushing everything into one app will be a major change, but the guidelines have been there for years; you could've looked at them and asked us this question before embarking on the apps. I know that doesn't really help, and you may not have felt that any of the guidelines would hit you this way, but you will gain a lot of knowledge in doing this, so it's not all been wasted effort.