I don't know how unwieldy (i.e. slow to load) that gets when someone has the full complement of available filters? Also, if collapsible means with javascript, then we need to think about what happens for people with javascript off, which I suspect is that all of both sets of filters show ... we need to make sure that, in that case (a) the relevant filters are on top and (b) there's a submit form at the bottom of each set of filters which will work for adding people to both reading and subscription filters, if you follow me.