Try pinning a flannel backed vinyl table cloth to a curtain or use it on a table. You can roll it up if you need to put it away at a moment's notice, then unfurl it later.

I always have more then one project on the go, however, only one at a time on the design wall. What is on the wall is what I am currently sewing...when I move to the next project I remove the first project and put up the next one.

It's important to keep organized, I pin in rows and # my rows so I know where it all goes...when I go back to sewing that project I have it numbered and ready to put back up. Because I've done it for so long it takes me hardly any time and as it's my routine, it's become automatic and I don't mess it up. I can go back to projects months later and my rows are in order and make sense.

On the rare occasions when I have a quilt on my design wall and need to put another quilt on it, I arrange the second quilt on my large cutting table. Like YukonViv, I number my rows.

I bought a second piece of flannel and pin it to the top of my design board over the quilt that I already have on it, that way it sort of makes a double board.