Another alternative is to use a battery operated switch for the auxiliary switch. The auxiliary then talk to the hub which passes the message over to the master, creating a virtual three-way. There are a couple of zigbee options for that on the buttons list, notably the SmartenIT 3 toggle and the Osram lightify smart dimmer switch.
The battery operated devices won’t act as repeaters, though, if that’s another feature you were looking for.