Hi.i need more explanation but  If forward/reverse primer has only one mismatch on your template according to the position of mismatch with original forward/reverse primer position, you may have additional amplicon (additional band on the gel) or not. It is better to check it with primer blast service at NCBI or other primer designing software.

The infuence of a mutation on annealing temperature (Tm) in strongly dependent on the position of mismatch in the sequence. Generally, mutations within 2-3 nucleptides at the 3'pirmer will greatly affect Tm while those in the center or a 5' end of the molecule have no or marginal affect. Although there are some programs to predict Tm of a mutated pirmer I recommend to run PCR at different annealing temperatures on a Gradient thermocycle when using mimatched primers.

I hope I understood your question right. Simply, if  you have two different primers with two different TMs for isolating one DNA fragment in the same PCR reaction,  you can simply use the lowest TM. You can always use gradient