Sounds like you need to make a virtual value list.
These links will help you out:
Basically you can calculate the values you need from the two tables with List() or ExecuteSQL() functions, and use one of the above techniques to turn that into a combined value list.
You'll need to reload the list every time you load a new client record context though, so don't expect it to be the greatest performing thing ever if you have hundreds of related records per client. If performance becomes an issue, you may want to store a list for each client and turn that into the value list instead of calculating it on the fly.
Thanks for that.
Another idea that I'm exploring, but getting stuck with, was to create another table CoverageMerge that is populated by both content and events automatically. so when I create a new record in either content or events a new record would be created in CoverageMerge, then my value list could be created from there. Could that work?
that could work, but you're duplicating record content, which always gets messy in the end.