I'm not currently uploading more than one comic on Tapastic, but I do have more than one project going - to keep me from getting bored/burned out on Grassblades. Thus far this year, I've managed one 7-page scifi oneshot, one 30-page silly romance oneshot, and am in the middle of colouring/finishing a 12-page scifi oneshot - on top of the 90-odd pages I've drawn of Grassblades itself.
And the answers to how I manage to do that are:
1.) Discipline. I sit down and work on my projects whenever I have the time. I get up early on the weekends (06.30-07.00) and set myself a list of things to do each day, and then I start crossing things off the list. I work early in the morning, I work late at night, I storyboard on lunchbreaks, etc.
2.) Planning. I storyboard an entire chapter of Grassblades in one go, so that I never have to stare at the next blank page and wonder what it's going to look like. This speeds up the process for that comic, and frees up more time for other things. The same goes for my side-projects; I storyboard the whole thing (if it's a one-shot, which it usually is; I don't know if I'd have the energy to keep two on-going comics at the same time), so that I can work on it whenever, without having to worry about making the big decisions mid-process.
3.) Self-imposed deadlines. This one goes together with the first two points; setting myself deadlines helps keep me on track.
4.) My work-life. I am a freelancing illustrator (hi please give me a job) working from home, which means I have a lot of advantages other people don't. My workplace is literally ten feet from my bed. I can switch between work and comics-making at a moment's notice, using the same equipment. And (unfortunately, in fact) it also means down-time between freelancing projects, in which there is little to do other than work on my comic and worry about money - and out of those two options, I'd rather work on my comic.