Apparently it's been going on for a long time, quite safely. Ben Edelman points me to the DRAFT MANUAL ON SIMULTANEOUS OPERATIONS ON PARALLEL OR NEAR-PARALLEL INSTRUMENT RUNWAYS (SOIR)
And Jerry Green gives the rules for Logan, with the aid of this airport diagram:
"There are a couple of combinations of runways that can be used simultaneously for landing. Ideally Logan likes to have three active runways: one will be for landing only, one for departure only and the other one can do both, depending on the wind and the types of aircraft involved.