In the group I'm in we put 2 large tables together but with very large quilts it's still not enough so we work on 1 half of the quilt and then carefully pull it over to pin the other half. And to keep the backing from moving, we clip it with large drawing board clips onto the border of the table, only where it can't be clipped we use tape. I've also seen some people use those clips you put on your patio table to keep the table cloth from blowing away. Hope this is of help to you. Good luck!!!
