From what I gather, unfortunately there is no way to combine two general/lords under one army.
So, this is causing another issue. I deploy my main army (army1) in a defensive position, for example behind a bridge. In Thrones of Britannia it does not allow armies to deploy directly on a bridge so I deploy them as close as possible. I place a smaller force (army2) behind army1 as a reinforcement. I want army1 to bear the brunt of any attacking force crossing the bridge. However, the attacking enemy army crosses the bridge, avoids army1 and attacks army2. Army1 reinforces but it's troops are not deployed in an ideal bridge defense position and or army2's general is now "leading". Is there any way to prevent this from occurring? If this is an intended mechanic - why?
Moving away from bridges. I think it could be interesting if there were some sort of "intercept" stance. If an army approaches within a reasonable distance of an army in such a stance it would attempt to intercept. This would prevent armies from simply marching around in an unrealistic turn based fashion.