Paul: your link results in a 404 message (page not found) to me.
Fixed - sort of. The link
was from Google, and it worked before, seems Everlight have recently repudiated the product.
would provide datasheets which could be found on Google instead of all the rubbish/ unhelpful "datasheets" sites.
Google helped with that though.
It is an AC optocoupler, which should also need 2 wires, doesn't it ?
My thought was that if for whatever reason (and it does make sense, because it would work with either a common positive or common negative to the group of opto-isolators) they fitted EL814s (originally, a Sharp PC814), then if the common is not connected, pulling one input high and another low would activate both optocouplers. Not necessarily deliberately, but would explain it apparently working with no common connection.