Please confirm on what platform and on what software version you are using the APs.
In mesh connection, mesh AP acts as an STA (client) and that means max bandwidth will rely on AP-STA compatibility and support. In this case, if R650 only supports 2 stream, mesh on 2.4G will be limited to 2 streams.
By default mesh feature uses 5GHz radio, but if you force it to 2.4G, it is not recommended because channel availability in 2.4G is very limited and there are only 3 non-overlapping channels. This means channel bonding can easily cause interference.