I was writing long before I got into filmmaking... so my workflow from short stories/novellas has carried over to my scriptwriting.
Basically I will have a number of ideas (which are all flashcards that are taped to my wall...) to pull from, normally the ones I've been thinking on for awhile will be the one I take down and being working on. I'll spend most likely two weeks to a month writing my first draft (this is a short film time frame) then I will put it in a binder and put the binder away with the rest.
Now that I've spent a time working on that project alone once I put it away I will pull out one of my PAST created scripts. This project I may have worked on from a month ago to a year ago. I can now reread this with a fresh mind and make changes. This will be the first draft editing to my second draft. I'll completely rewrite this script with it on one monitor scrolling along with my new writing. Most likely a lot of it will transfer over, but it will change significantly as well. At this time, I will hand it over to a screenwriting friend who will clean up my dialogue and make it more realistic sounding (as that's my weak point). He'll give it back, I'll read it and then put it away again.
Now I repeat the first process with a new idea, making another first draft for a project. Put it away. This is where something big comes. It's most likely time to take out a second draft from my binder pile. This is now going to be reread and given a FINAL draft. Then will immediately go to pre-production of storyboards, shot lists, character bios, etc, etc, etc... This process will most likely take a month at least. I go VERY in-depth with my pre-production as the better you do here, the better it will do in production. I promise.
Then I'll put that final draft and pre-pro away. Pull out a binder or start a new one. Eventually pull out my final draft and pre-pro again, give it some touch-ups and then build my crew to go into production.
So never am I working on two projects at the same time, but I do have a lot of shelving to keep my ideas and viewpoint fresh.