With a bit of lateral thinking it is possible to achieve multiple add to carts with quantities, this is one of the mods that I have done on a few sites Cotswold Kitchen & Bath, Murranji Photography and Red Dirt Photography
It involves setting up a unique session for each customers shopping session via php's session_id() then insert the selected products into a temporary table with Data Assist's Multiple Insert Behavior, create a recordset based on the session_id() to draw out any products selected, use eCarts Get Info from Recordset Behaviour to put the products into the cart from this recordset and then delete the contents of the temporary table before taking the customer to the view-cart page.