Do their paintings ever overlap?
Do they protect their own space, or see the who page as theirs?
When they are through, label the picture, "This paper was shared by Jason, Brian, Kendall, and Scott." Parents can also comment on the fun of sharing.
Instead of taping the paper down, you can simply use large clamps like those designed to close open bags of chips to fasten the paper to the tabletop.
While children rock back and forth together they are learning that it is fun to be with other children and do things together.