It's not that dangerous of a combo since meth acts primarily on dopamine and norepepherine so serotonin syndrome is unlikely, but you will probably get a diminshed feeling of euphoria.
I noticed an increase in the likely hood of extreme maniac behavior when using stimulants while on SSRIs. I can't speak specifically to methamphetamine but I'm sure it would have been the same for me regardless of the stimulant.