We do something similar with our modular helmets - we put them on the seat and run a single rubber coated cable through both helmets and a passenger grip rail and use a padlock to secure through the end loops.
If it rains while we are out walking or whatever, the insides will not get wet.
If we will be gone a while, we throw the half cover on the bike and it helps conceal the helmets also.
Once you have had one helmet stolen, you learn to always lock them up.