i think you can use way points for this have a WP close to your invisable H pad for the second heli before it lands, then have a WP for the frist heli after it takes off
then press F5, hold your right mouse butten down on one of the WP's and move it to the other WP. you will see a blue line, "sync" them together, this way chopper 2 will stay at the WP untell no. one heli reaches the other WP w/ the blue line,,,, i hope you got me on that,,,,,, someone else might have a better idea, cheers