So in summary;
If the force from the rudder is equal then the one obtained from the bank, while at the same time you balance the moment from the live engine, you have zero slip straight flight.
Whether this is at all possible depends on the type of aircraft and/or you may have to do it with less than max/cont power/thrust.
Hereby you will have to neglect the small force contributions from ailerons and spoilers.
In my opinion the zero slip straight assymetric flight will have a deflection of the slip ball equal to the bank.