You mentioned that when they get bigger you would just remove to divider. But I dont agree with housing two bp's together like that. I think BP's should always be seperated for reasons mentioned above and the risk of one dominating the other and even eating the other (though this is not common). And a 20gal for two BP's is much too small in my opinion. Maybe you ment seperating them at that point and keeping one in the 20 gal?