It's been mentioned in the Viking sagas that warriors would use slings to carry their shield on there backs outside of combat as well as in combat were they were they wished to use a weapon with both hands. I have seen this style adopted by reenactors who use use spears, long axes or a combination of two weapons and I have experimented with it myself in broom handle dules with my friend taking turns being both spear man and swords man.

What I found was it is most effective when your back is to the side were an outside blow will come from. Most of the time the shield will absorbe a blow from the outside while you can use the spear to parry close range blows from the inside. It also gives you the luxury of turning into a blow to deflect it while you spin away to regain ground for a counter attack . This can also be aplicable two other weapons such as axes but it work most sucessfully with the spear. I see this as a very useful technique to use but does it bare any hisorical fact the only reference I have seen is in the sagas

If you look up Viking Shield in Archaeology you will find mention of rivets and possible rings on the periphery of shields. These could have held straps that would have held the shield on the back during travel.

The use of the slung shield in combat is known from the ancient Greeks, but in terms of the Viking Age, it is surely a "reenactorism".