Sounds like Employee “A” is a bottom feeder. But a remedy to your situation may be:

Option 1. Request a per diem contract with Employees "A" and "B".

Option 2. If Employee "A" wants 80% of your salary of Employee "B", you can work out a salary/per diem arrangement with Employee "B" where your salary is equal to 10% and per diem equal to 90% of your total pay from "B". Then you would be obligated to pay 80% of Employee "B"'s 10% and also advise him that you would issue Employee "A" a 1099.