I use a repeating trigger with great success.
condition
helo1 distance helo2 < 50
activation
helo1 setpos [getpos helo1 select 0, getpos helo1 select 1, (getpos helo1 select 2) +50]
If you see the trigger activate, it won't look so great, but it's better than having them collide and ruin the mission. Newer versions of OFP have improved anti-collision for AI units, so it may not even be necessary if you have resistance version.