They should remove strength for co-op all together...for one a player can already solo a siege....the co-op is more for a social experience aswell as a mentor experience or introducing new players to the game and helping them out.
My homeland being advanced and armies high past 1k+ and my brother who just started has a 135 army. So ok I thought np i'll just take a few siege equipment and my hero, so I make a 135 army aswell.
We targeted Denwell to attack and first he tried, it showed hes army at like 60 something and denwell at 90 something with the co-op rating being 40 in the top right hand corner of the icon of denwell. When I tried to siege denwell with my 135 army it boosted denwell to 400 something.
And we could not get the game to match us together.
Hopfully they will work something out for us :P