Double ownership

Double ownership

Hello all,

In our company all the external sales people have an internal sales person as a back-up/teammember.
So for every account there are, in reality, TWO owners. Both of these should be defined at the level of account module.

I would have solved this by creating a lookup field searching for "users", but this does not exist.
Does anyone have another solution (other than creating a picklist with all usernames)?

Thank you!
Grts,
Shanti