I'm not suggesting that placing the DNS-345 in Active Backup mode can be used to connect the DNS-345 to two independent subnets, but am rather stating that link aggregation is enabled in the "Active Backup" mode by default. This being the case, resetting the DNS-345 to the default settings (as you suggested earlier) would not disable link aggregation
I believe that "Active Backup" only enables one NIC at any given time, with the second NIC serving as a fail over if the switch attached to the first NIC fails. Any flavor of link aggregation will only serve to provide fault tolerance, increased throughput, and redundancy.
More to your point, I don't know of any way to disable Ethernet bonding on the DNS-345.