How create a Select List (single select) showing values from two lists concatenated together?

[Gillian Martel]

I have two lists of values Projects {a, b, c} and Overheads {HR, Support, Facilites}. I want a user to be able to select any value from either list. Each list is managed by a different person so it suits us to keep them separate. 

I know that I could create a field for each list and do some validation to check that a value from one or the other but not both is selected. 

How do I go about initialising the values in a Select List (single select) field that I can use on my Jira screens? 

Thanks, 

Gillian