2 lists into 1 dropdown

[RT]

Hello forum, so a question.

I have an expense table (z_expense ID)

I have an employees table (employees ID)

all is working fine i can populate the dropdown list from the other table.and it fills in the (firstame) and (last name) fields now to my problem. i also have a staff table (staff ID)

what i would like is when i click expense ID, all the names would be listed (drop down) from both (employees ID) and (staff ID) so i can select my contact. been thinking but not learning fast enough at the moment. :

thanks