i wouldn't even worry about a second inlet on the grinder. not enough rides the belt to worry about. i have to do a bunch of grinding to get any dust to build up on my bench.
i have a second inlet under my disc grinder. each inlet funnel has a slide gate (shutoff) so that i can maximize the suction at the machine i'm working on. that's another reason not to have extraneous inlets around the grinder- it would rob cfm from the primary funnel.
i originally intended to blow the dust right through the wall and forego the collection bag altogether. I still may do that, but i hooked the bag up to buy myself time to make a port through my wall and was shocked by how well the bag worked. It worked so well i never got around to busting a hole in the wall.
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