Yes, as long as the defending unit(s) are in all of the attacking units' frontal arcs then they can be combined in the attack. Please note that per 9.1.2 and 9.1.3 all attacks and attacking units must be declared before any are resolved.
Two more came up as I worked through Narva
1) Where do Russian routers go once the bridge collapses?
2) With combat resolutions, what happens when the final result is a tie?
I just treated it as a "no effect"