Is it safe to re-heat food twice in a short time span?
The real issue is the total time it spends in the "danger zone" (40-140 F, 4-60 C), not how many times it is heated, frozen or refrigerated.  Those things may change the quality and make the food unpleasant, but that is a separate issue.  Re-heating in and of itself is not inherently unsafe.

See also:

How do I know if food left at room temperature is still safe to eat?