I'm using a similar setup myself and I think a lot of single class multi-boxers (can) do the same. To me it doesn't really matter which character is the current focus char, but I usually do end up with the same char having focus (acting as a main) because it simply is the char that at startup gets assigned to the main window region. For pvp I would even recommend switching chars a lot, because enemy players usually find out very quickly which character you are controlling.