How different can the Tm be between 2 primers?
The default parameter provided by "primer-blast" is 3 degree (max difference between the melting temperatures of the pairs) .
that said I think it really depends on the application. in case of cloning (in my experience) you can allow wider ranges of delta Tm between primers (in this case I used to use the average Tm of the two primers as annealing temperature). you accept to have a bad efficiency but eventually you can pull out your insert from the gel and clone it in a vector for a prep.