Is the reward for a funeral prayer multiplied if there are multiple funeral prayers at the same time?

Yes, the reward multiplies with the multiplicity of funeral prayers. The apparent meaning of the hadith is that the one who prays over each funeral receives a Qirat, so the reward doubles with an increase in the number of funeral prayers. Thus, whoever prays over one funeral receives a Qirat, and if he follows it (to the burial), he receives two Qirats. This is from the grace of Allah upon His servants.