For asymmetric ReferenceTypes RC2 defines the following: "All other ReferenceTypes shall be represented by a pair of AML InterfaceClasses where one element of the pair represents the source endpoint, and the other represents the destination endpoint corresponding to the forward and inverse names. "
In practice I expect that it will be quite hard for tools to implement all the connection rules and to even associate the forward InterfaceClass with it's backwards companion.
The currently specified solution is nice for humans to read but not ideal for software applications.

It is also only implied that a forward Interface can only have an InternalLink to a backwards Interface.

AutomationML provides standardized Attributes (Cardinality, Category, Direction) that are supposed to solve the problem of what can be connected to what in a generic, machine readable fashion. FX should use these Attributes.
Cardinality - must be used to describe the allowed maximum/minimum connections from/to this interface.
Category - Only interface Classes of the same category value are allowed to be connected.
Direction - In, Out, or InOut. Interfaces with In can only be connected to InOut or Out, Interfaces with Out can only be connected to In and InOut

If these attributes are used it will be much easier to use standard AML tools to check the validity of FX AML.