Access user control properties in another user control

I want to use a couple of instances of the same control on a page.
The user control is intended to display a list of records in a repeater. The aim is that the second control will show a list excluding the IDs in the first control
The control has a number of public properties, including IDsInThisList, and IDstoExclude.
The query will exclude the IDs in the second property.

What I would like to do is get the record IDs from the first instance of the control (IDsInThisList), and pass them to the second contro as the IDsToExclude.

I have a test page where I am setting the properties of the controls in code. As you might guess, it's not working!  
The first control displays the top 3 items, as desired, but the second display the records 1-7, rather than records 4-10 because it is not receiving the IDstoExclude from the first control
When I response write the IDstoExclude from the first control in my testpage, the string is e ...