[AccessD] A2003: Design Question

Hi Team
I Have a dB that holds data for both Clients (tblClients) and Franchises
(tblFranchises)
 
The tblClients holds all the usual client detail stuff - ClientID is the PK
(Autonumber)
tblFranchises holds various bits of information about any particular
franchise - FranchiseID is the PK (Autonumber)
 
So essentially 2 entity groups within the dB.
 
BUT my client does work for both the Franchises and individual Customers. 
In order to present both of them in a TimeBilling Screen I use a union query
to 'merge' both 
groups into a single combo
 
The problem is storing the PK correctly. eg say I select the 4th Franchise
in the list - it has a PK of 4
Well an entry on the client side will also have a PK of 4.
 
I am tossing up with various ways to deal with this including merging both
tables into one
table and just setting a flag to differentiate Clients from Franchises.
Of course I can have separate fields in the tblTimeBilling for FranchiseID
and for ClientID.
 
Just not sure what would be a sleek solution :-)) 
 
I am sure this is a common type of thing - I just wanted to know some
methods on 
how you guys deal with this sort of design question.
 
Many thanks
 
Darren