Old school DC adaptors with iron transformers, rectifiers & filter caps did not have the same issue - the rectifier diodes would alway act to effectively short the output if sufficient reverse current were applied.
If you just add 3A diodes ( ie 1N5401s etc ) to each SMPS adaptor's output you will probably be OK.
A Schottky rectifier from each rail to common will usually save the load from dying due to supply reversal. Otherwise, if the outputs are isolated, there should be no issue at +-12V. I'd do it myself in a heartbeat.