The above process seems a bit strange that you double clicking on a combo
box to display a list of customers? does not the combo box already have a
customer list that you display when you click and open up a combo box? I was
thinking that you could double click on the combo box and thus open up a
form to edit the customer currenlty selected in the combo box.

For some reason you seem to want to be able to do the reverse process?

You'd have to use continuous form, and when the user double clicks on a
field in this new form, you simply use the double click event of that
control, and stuff the value of the customer id back into the combo box.

forms!nameOfFormWithComboBox!NameOfCombobox = me!ID

You could also consider putting the above line code in the close event of
this form (but, that would not allow you to have a cancel ability).

For the most part I would've thought the interface works the other way
around: You select a customer in the combo box, then double click of a combo
box, and you open up a the customer form that lets you view/edit the
customer. It really doesn't seem to make sense to select a customer in a
form, that sets a combo box and that combo box already has a "list" of
customers in it that you could have selected by simply dropping down the
combo box.  Why open up a form here???

Anyway the above code is how you can set the value of a control in a
different form from the one you are in...