SCSI cables and Mac serial cables are probably the most plentiful old-Mac-compatible item at thrift stores. Well power cables too I guess. I actually found one recently that had a SPLITTER ("SCSI Tee") attached on the 50-pin side - 1 male and 2 females. That's pretty cool. Though it may be more useful to have a splitter for the 25-pin variety, it is theoretically possible to make this connection, whether it would actually work or not:

Mac -> 25 to 50 cable -> splitter -> typical external scsi device with 2 50-pin ports

Other female 50-pin port on splitter -> 50 to 25 cable -> Zip drive #1

Other female 50-pin port on typical SCSI device -> 50 to 25 cable -> Zip drive #2