As has been discussed, I try and do what I'm told.
As a mostly sub-V, AAU and Rec Official I use the whistle to bring in subs as the table crews are oft times not stellar performers/volunteers. Now, as I get better table crews at the JV/V games, often the horn will actually sound, in which case I have been instructed NOT to use the whistle as it is redundant.
I don't do College games, therefore I never use two-hands to report. My thoughts on that - I'm kinda in Nevada's camp here. Hey, if you do both levels and you can keep both sets of rules straight, you can keep your hand signals/mechanics straight too and you KNOW you should be using one-hand.....so, why use two? Unless you're trying to show someone something.....and what could that be?
JMO