FrenchCreek Production dual leg "pack" style web lanyards allow you to remain tied off at all times (100% tie-off) while in transition to other locations or positions.  Example:  When both snap hooks are connected to an anchor point, simply move one connection to a new anchor point, then once it's connected, advance the second snap hook from the previous anchor point to the new one.  This will ensure 100% tie-off at all times.