DC fan current requirements
There is a large latitude in your definition of "run". Were you conducting your test under load?  (Where they are sucking and/or blowing against the proposed "load" of the final installation?)  Testing in "open-air" is not the same as testing under load.  It seems quite possible that two 1.9A fans could appear to operate in a no-load, open-air "test". Was the RPM of the fan the same when you tested it with only one fan?  (i.e. was the power supply output "drooping"?)

It is also quite possible that a multi-output switch-mode power-supply (SMPS) could tolerate one of the outputs being over-loaded in cases where the other outputs were not approaching their load capacity.

In any case, it appears that you have a 12V power source that is not adequate to the rating of your two fans. You could use a separate 12V supply for the fans, or you could use mains-powered fans and avoid involving the power supply.