The Y cable supplied with a legacy base is expressly for the purpose of connecting and original base 1 to your legacy base. I do not think it should be used in your configuration. Take a look at the SER 2 manual and review the different examples there.
I have not experimented with the MTH product, but I would recommend that all connected serial devices are fed from the SER2. Perhaps a second SER2 might be necessary to connect all the equipment you have, but in any case the Y cable probably is inappropriate for this configuration.