There are three ways that I can think of.
1. Set a button to 'SHOW RECORD' and make sure you assign the second combobox to the correct foreign key.
2. Possibly (I haven't tried this), set the first forms combobox as the parent of the second forms combobox.
3. Use script as in the following:
procedure frmEditPart_Button1_OnClick (Sender: TObject; var Cancel: boolean);
begin
ShowRecord(frmAddAltSupplier,'productsupplier',frmEditPart.grdEditPart.dbItemID);
// testsql.show; do not use this line
end;
The format is ShowRecord(Form you want the info to show on, table name that contains the data as a string, form and component info is from), in your case that would be the first combobox and it's ID.
For a combobox I think you would put the code in either the 'OnCloseUp' or 'OnChange' even of the originating combobox.
Edited to comment out tstsql.show - this was something for me that I copied across by mistake.
On a clear disk you can seek forever