just add the same amount of each primer as you use in single pcr. The magnesium and all other reagents including the template are in excess and should not need to be changed. There is slightly more chance of a primer dimer forming and if this does happen then use less of each primer

Paul Rutland can it be like, I am adding 1ul of primer A, 1 ul of primer B and 2ul of primer AB. is this ok

It is not possible to work with volume amounts .Absolute amounts are more accurate but if you have found that 1ul of your primer dilution works in each single amplification then I would use 1ul of forward A, 1ul of forward B and just 1ul of the common reverse primer. Primer amounts are alays in huge excess so just 1ul of reverse primer will be plenty. Of course you will have to use the temperature of the lowest annealing primer set for your mixed pcr.