Do you have any recommendations to make this project run successfully?
One of the questions is what they would draw, and what tools could they use. The could be drawing using various brushes, with multiple layers and transparency. I suggest using a basic javascript controlled brush system, and the users' browsers can share the brushes and the mouse paths ...
Proposal:
(input) so the results could be generated on both sides. Communication can happen through web sockets. Brushes could use images and all, or they could be procedurally generated (through javascript code).
But then what does it mean for two artists to draw together, could two people be working on the same thing at the same time. I have not seen people doing the same thing together. There is usually a division of work method. So, instead of drawing together the artists should be able to work on different parts of the design together and see each other''s progress realtime. Possible there has to be a concept of PARTS or MODULES. Lets brainstorm together.