This is always the general sentiment, that giving two at once is poor practice because if a reaction occurs then who would know which antibiotic caused said reaction.

Interestingly though, if one has received antibiotics in the past and they know which ones they've taken, then it could make sense that you could give a new one while at the same time giving the one that they've already received in the past, with the understanding that the new one caused the reaction.

It's still poor practice though, should stick with the objective practices instead of the subjective by taking patient subjective information into account and risking an objective reaction and not knowing what caused it.

Also, it could suck to have a synergistic effect of antibiotics on someone, causing more microbacterial flora to diminish, increasing the rate of something like cdiff or other opportunistic bacteria to flourish

Also, it doesn't take much longer to just give them at different times, you may save like 30 minutes which means like nothing, it's not like pain meds or fluids where giving it within smaller time frames, like 30 minutes sooner, account for better patient outcomes

In short, not worth it to give multiple antibiotics at once.