as we don't supply any interoperability with ISA directly this would be a question in traffic routing etc. You could also think about using an external loadbalancer to fullfill this taks. Generally I would suggest to use an active and a warm/hot standy for MWG. Why not having two MWGs in HA and in case one fails, it will automaticall only use the remaining one?
Michael Schneider Lead Product Manager for Web Protection (•‿•)