I recommend the blocker ties for training to stand tied. If you don't want to buy one I did used to use an innertube looped around a tree then a long lead or lunge line run through the tube. Either way you can let the line run out if they freak, and you get the release. Don't
go off and leave them alone. I will throw the rope over and around them and if they try to pull back just let them do it, praising them when the stand quietly. Don't hard tie them till they can stand for long periods without freaking. I work my way up to having them stand tied while I drive past them in the truck or 4 wheeler, but that takes awhile
As far as the other youngin, do you have someplace you can rig up that he can't get out of? You will have to separate them sometime so best to do it sooner than later. Maybe start taking one out on a lead out of site for a few min. Then taking him back. They need to learn that even though their buddy is gone they will come back.