Walking two dogs- leash pull

Question
Hi
I recently rescued two one year old labs.  They are great dogs.  They walk almost perfectly by themselves on heel and a loose leash.  When I walk them together, in a situation which is new or might have good things like lizards, my female lab just puts her head down and pulls.  I tried changing direction- but then it seems to punnish my other lab who gets it.  After a few changes he is right at my side.  Both dogs have gone through obedince training with a private trainer.  But my female is soooo stuborn.  She generally heals well in this situation.  But we are going backpacing this summer and I want her to be able to walk a comfortable distance ( 4m) ahead.  
Thanks Lynda

Answer
I have never done well walking 2 dogs at once.  They seem to know you can't use you usually corrections and take advantage of it.  Have you tried a head collar?  The leading brands are Promise,  Haltie, and Gentle Leader.  They have a strap going around the dogs nose looking something like a muzzle.  They work by pulling the dogs head around.  No other way gives you such great control with so little force.