I don't know if adding two points of failure (my heirs and the executor) is a good idea.

Following your idea maybe I would use a different technique: give my heirs the seed in an envelope with some particular photo/graphic on it and tell them they will find the "other part" when they will review my possession. And then simply custody in my home an envelope with the same photo/graphic. This would make obvious the connection only to who own the first one.

Another idea is to zip encrypt the passphrase using the seed as password and put it on some usb, asking to 2/3 friends to give it to my heirs. Of course they should be informed that have to use the seed to retrieve the passphrase and then use them combined (more details can be stored in the zip itself).