IMHO tip up is easier with either hand on a LBV or lifejacket, or pack strap....as long as the knife is mounted higher than mid-point on your chest. If you mount your knife below your sternum, tip down is easier with either hand.

A sheath properly made for tip up carry shouldn't cost you a knife if it is used properly. Many (sometimes called "jump certified" or "para-certified") have secondary fasteners, too.