I suggest not to use enumerations.
Create the connection to the 2 systems to read the data, maybe via a REST call to the systems. Make sure to store the data in 2 entities, one for system 1 and one for system 2 with 2 attributes (site name and project name) in each entity.
Create a third entity that has a 1 – * associationto entity for system 1 and the same to the entity for system 2.
Now you can create a page with 2 reference selectors on it using the created associations and the user should be able to create a new record in the third entity and create the connection between the 2 records.
If more automation is needed the result of the 2 REST calls (the data in entity 1 and 2) could be used to loop over and automatically create the record in entity 3 based on a retrieved action via the name (if this is uniquee enough). If this is an option then entity 3 would not even be needed as an setting an association between entity 1 and 2 would then suffice to create a grid with the data from entity 1 and 2.
I hope this gives you an idea on how to implement this requirement.