During Medium Events, schemes are grouped up in pairs! Upgrading one scheme also benefits the other scheme in the pair!
As an example, here both Paddy's Pub and D*ck Towels each at level 1.
Both of these schemes are paired together. When D*ck Towels is upgraded, not only will there be a x5 multiplier for D*ck Towels, but Paddy's Pub will increase as well.
Even though only D*ck Towels was upgraded, the earnings from Paddy's Pub tripled from $70.40 ad per second to $211.20 ad per second.
One thing to be aware of when upgrading: the animation only shows for the one. If you watch your cash totals carefully, you'll see the paired scheme increase too!
Checking the store is a useful reminder on which schemes might be paired.
Note: Only schemes are paired, not characters.