Running 2 or more molds is not something I've ever got the hang of or cared for. Nothing wrong with it, just not for me. I will abuse a Lee 6 hard, but do enjoy the heritage, & consistency a single offers.
Almost all of my casting gets done using two moulds at once, usually 2-cylinder models. I agree, better production and more keepers than with the 4-bangers or Lee 6-cylinder tools. Whatever works, though.I usually get slightly higher production from two double cavity moulds than I do with a similar design 4 cavity. If I end up with a two cavity mould that I really like I'll often purchase another identical mould rather than buy a 4 cavity of the same design.