Hello Microburst2002,
I do not recall anybody telling me about the importance to select or not to select both FDs simmoultaneously. Only to select either both or none, as stated in the manuals.
I encountered A/THR transient failure only once after take off. Don't know if FDs simmoultaneous selection contributed to it.
Anyhow, I don't see why not changing my habit and select the FDs ON with a short intervall. I like the "one at a time" theory better, even if it is just for FDs on the ground.
Regards.