I have to aggree with Daniel. Your network topology looks a bit strange.
Not only is there no master in network 2, but I also think that the on-board DP port on an S7-300 cannot be used for master-to-master communication (I think that the S7-300 cannot initiate a datatransfer, but it can respond to a data request from another master).
You could switch the AB over to the CP card on the S7-300 - that would allow master-to-master comms, and you could connect the drives on the on-board DP port set up as master.
But the neatest is to set up everything in only one Profibus network with the S7 315-2DP being the master and the AB one of the slaves. It looks like that the SST can work as both master and slave at the same time (using two node addresses I presume).