Connecting two SMPS in series
Your 12V DC supplies are not 100% equal. Another starts sooner than the other. Please draw for yourself a connection diagram of the outputs and the load. 

See it in a new way: One power supply feeds another with reverse polarity voltage through your load. Maybe that's noticed by the safety circuits and considered as a proper reason to stay quiet.

Few millisecond delay between the mains voltage phases gives the right head lead for the slower.

Probably your system works from one phase too, if you let the PSUs start in the peace and connect the load thatafter.