i can double confirm that.
the guy who owns one of my castles is also the guy i use to jump from shard to shard with my shard shield tokens almost monthly.
never had a problem.
its a account based thing so it really dosnt matter what name is on the house.
another example is when i transfer to another shard i can take that transferred guy and just walk right into my other houses because someone else from that account is co-owned, even though that fella is from a different shard its the same account so he is covered.