After Update

Why do you need two if they are the same value?
Are they ever different?
Could just bind it to UserID.

As an aside are there always 10 employees and 10 inmates?
You might want them as subforms so you can have any number.
Relationships

@ Alex The "UserID" is an autonumber assigned by Access on the UserT. essentially they are the same I suppose but my table is not updating with just the info being in the "Assigned Investigator" box.

Also, thanks for the side note, there are not always 10 of each, but they differ from time to time. I will give that video a look soon.

I am trying to get Kevin's suggestion of using the After Update event to get the form to do what I want, and I am almost there. I have it set up like so:

Private Sub Combo15_AfterUpdate()
  If Combo15 = "1" Then
      UserID = "1"
  End If
End Sub

This works for the first Investigator named in the UserT, but no others.... any advise?

Should also note... in the final version of the form displayed the "UserID" field is not visible.

You are dealing with IDs (numbers), not strings,  so you don't need the quotes.

Private Sub Combo15_AfterUpdate()
  
  UserID = Combo15
  
End Sub

Be sure to give your Combo Boxes (and any other control referred to in code) descriptive names.