There's no technical reason why you can't if you use active DHCPv6 and other such tricks. They make life harder than standard IPv6, but there's no reason why you can't turn one of your /64s into /120s. With a /56 being the standardised assignment for ISP customers (=256 subnets), I think there's more than enough space for a home network. /48s used to be the recommended space assignment but I think these days you can only get those as a business customer or through an independent party (i.e. Tunnelbroker).