either i'm not understanding what you really need, or perhaps you don't understand networks ?
is the "double ethernet ported arduino" just being as a daisy chain to extend the network ? as in you dont have spare network ports upstream of it ?
if its really bridging, ( as in crossing two network ranges, then why dont you open up the network mask ? ie if one ( or more ) arduino's are on say 192.168.0.x and others are on 192.168.1.x then you dont need to bridge the two, you set the ip's as needed ( the 192.168.(0 or 1).x ) and then use a netmask of say 255.255.254.0 that will work, if some brain dead ethernet library doesnt like that, then go down to a 16bit mask. ie 255.255.0.0
have you read / understood how net masks can help you ? i'm struggling to understand why you need separate networks and to bridge when clearly they must be within 105 meters of each other ?