I used to upload two at once, my main serie, 6,75, which is a very long fantasy comic, and Trust Me, I'm a Chemist, which was a silly comic about my memories from studying chemistry- at least it started like that, because later there were also episodes not related to chemistry. Updates schedule changed with time, but I think at my peak I was uploading 3 pages of 6,75 and 2 episodes of TMIC per week. I'm not sure though, might've been 2 / 2.
I think it was possible only because those comics are so different. I don't think I would be able to do two bigger series at once and put the same amount of effort into them
6,75 is much more detailed, while TMIC is kinda just doodles (but was WAY more popular anyway haha)
and this way I could rest from doing one serie by drawing episodes for another one
and the key was to draw comics daily, even just a little bit, and doing a bit of extra work when I have more time to have some bigger buffer
I could manage it quite well when everything was okay irl, however it was already to much when things in life got busy and I could no longer keep up with it. I still tried and got burnt out very badly cx but that was mostly due to the outside stress, not comics themselves. So my advice is to put one on hiatus as soon as you know you won't have that much time, and don't rush back to it too soon 