Official comment
Appreciate this post and the use case.
We wouldn't build this feature the way you're asking for it, because Ontraport is a platform not 'an app'. We're trying to build something that can be used for many use cases, not just a one.
The relationships between contacts, deals and companies are simple custom object relationships which, currently, are unrelated to each other.
In order for us to build something that fits your need, it would need to be something like 'Make it possible to filter the records in a relationship dropdown (deals) to only show records that are also related via another relationship (companies)'
That's an interesting idea. We'll keep an eye out for other customers who have this need.