是的，主句和从句可以不对称。主句省略了系词 is 或will be.  him or her tapping on it and the manufacturer making money是并列的动名词复合结构作介词of的宾语。主语过长，采用倒装结构。

The longer an advertisement stays on a user's screen, the greater (is/will be) the possibility of him or her tapping on it and the manufacturer making money.