I do something very similar, 2 x VDSL modems in the front room trunked back over 1 CAT6 port (internal wiring) to the rack under the stairs. I run each modem with 2 VLANs, WAN and management, using dot1q.
However, rather than load balancing I opted to bond both connections with an L2 VPN back to a box in the DC. I run each WAN in a separate OpenBSD routing domain, so that there can be a discrete default route per routing domain, then an OpenVPN L2 tunnel in each routing domain back to a box in the DC - VPN bonding basically. Each OpenVPN tun interface is then added to a round robin trunk interface in yet another routing domain - the trunk at the remote DC end is bridged with the external interface so I can pass a /29 from the DC to the router/firewall at home.