If Team A has the ball and both A1 and B1 go offside simulataneously, how would this be handled ragarding fla-down vs. whistle blown and penalty assessment? Thanks for any advice.

If the order was not know blow the whistle to make the ball dead, then call both teams offside and team A keeps the ball and neither team goes offsides. See 7-6-2-a and 7-6-3-b-1 for rules references. If the ball was on the ground resolve possession via the A.P. arrow as explained in 7-6-3-b-2.

No arguments from me. :)