I myself have done full-length simultaneous projects by myself and again with assistants (full-length meaning each episode is 50-ish panels, due every week).
I definitely will not be doing it again 
I found out the hard way why artists can only take on one full-length comic series at a time.
It demands waaay too much time and energy, more than you'd expect. Especially if you have contracts, editors, and hard deadlines involved. I wasn't able to do my best in any of the series and not only did my art/writing suffer, but so did my physical and mental health.
That said I AM currently doing two comic series at the same time (one main series, one short spin-off series), but I've staggered my updates and workload to suit my own needs so it's much more enjoyable + less stressful.