It just takes a lot of training. I'd suggest you work with one dog at a time and get them to listen to you before venturing out with two...
They have to learn to 'watch' you for commands...this can be reinforced with food rewards and lots of praise...eventually taper the food rewards out of your training...the walk itself will become reward enough...
I don't know what others do, but I alternate 'heel' time with 'free' time...I usually end each session on a sit to break them up and so the dog knows a change is coming...I also greatly vary the amount of time I spend in each phase to prevent anticipation...this eventually could evolve into a wee bit of free time just as you're starting out...and maybe a bit more when you're back home for example, so your entire walk consists of 'heel'...
It depends on what you want...I seem to have a problem with Celeste's chomping and mouthing

...but she's heeling really well!!!

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