The issue is that in a twin engine jet (for simplicity), it's unlikely that the two engines would be producing exactly the same amount of thrust at the same N1 or EPR.
Some aerodynamic surface(s) would thus have to be deployed to keep the sideforce zero, which would mean extra drag.
How does sideslip get measured (the yaw string is a good idea, but needs to be lit at night...), and what can be done to ensure minimum drag?