Request: Assign probabilities to groups or channels

Hi there,

I have an advertising spaces that I want to share between various advertisers. I am going to charge based on probability per advertiser, payment is on a monthly basis. But some advertisers might only submit one ad, others might have 6 ads in their campaign. But I need a way to assure that if I have two advertisers who are both guaranteed 50% exposure, so that if advertiser#1 only has one ad, and advertiser#2 has 6 ads, I need to guarantee that advertiser#1 will appear on 50% of the page views, and ANY ONE of advertiser#2's ads will appear on the other 50%. The current arrangement of assigning probability to individual ads doesn't really work well for this.

Thanks for considering this!