I have a goofy scenario (I think). We have a flat bridged infrastructure network that we run a routed PPPOE setup across. All of the infrastructure equipment is bridged (Canopy/Ubiquity). All of the clients sitting on the network are using PPPOE clients built in their modems and are routed through border/gateway PPPOE servers sitting on the edge of the network.
We are trying to install redundant links from all of our towers to increase our capacity and achieve fault tolerance. We are using Microtiks at all sites. At our first site, we have (2) Ubiquity PowerBridges jacked into 2 ports on an 493AH using v5.6 (same setup on the other end). We have added a bonded interface to each and chose balance-rr and assigned the 2 ethernet ports the Ubiquity gear is on. The link works and we can see traffic evenly spread across the two links. If one fails, the other takes over and it all works like a charm with one exception, were not getting double the throughput as expected. Both links test out nearly identical and give 50x40. We expected to see something along the lines of 100x80.
It was suggested to me that the reason we are not able to take advantage of the double throughput is because we are not actually “routing” traffic through the Microtiks with the bonded links, as for us, that would not be possible without tearing apart several other networks that also sit on top of this bridge network.
Can anyone confirm this and save me from having to buy a bunch of gear to setup a lab? Ive disrupted our network a bit too much this past week on this issue and cant monkey around much more 
Any help is much appreciated,
Tim