Spefically across two systems: can the domain of system A call the application services/remote facade of system B?
For example if an Ordering System has an Order entity in its domain, should the validation method of that Order entity call an applicaion service of a Stock Keeping Service to check there is enough product in stock to fulfil the order?
My instinct is that this is not the correct way of doing things.
This is a simplified version of a previous, rather convoluted question:
Integration between various Domain Driven Design systems
It is not nessacery or advised that you refer to this previous question.
First of all, it would pay to consider your architecture. Why is it necessary that your domain logic hooks into another domain directly? Can it not consume data that the other domain publishes? If not, are we really talking about two disjoint domains, or are they really part of the same bounded context?
It may be that the other system is a closed one, so that you cannot extend it. It may also be that that other system does not publish its data based on events, and that you can only access it through an API. In that case, it would be best to use an agent in your domain model. That is, your domain logic should contain an agent (proxy) that connects to the remote system but acts like it’s local. This encapsulates even the fact that the actual logic/data is not local.