Do not connect the loops with a switch for the inside rail. Common (U) on the ZWC will go to the outside rails of both inner and outer loops. Connect the A channel to the inside rail of one loop. Connect the D channel to the inside rail of the other loop. Separate the two loops by using a fiber pin on the inside rail so the inside rails for each loop are NEVER connected. If you want to run conventional, once a train passes from one block to the other, you use the corresponding handle to control the train. There will be a minor spark as the train crosses from one block to the next but the transformer can handle this.
If you were to wire in a connecting switch between the two middle rails, you run the risk of connecting them when both channels are powered. This could blow the transformer circuit board.
The ZW-C has a feature where you can configure it to have just one handle control all four channels. If you set it in this mode, you can just use the one handle to control all your blocks. Then running conventional is really easy if you just want to run one train through all the blocks. I would use this feature instead. Turn the feature off when you want to run two trains (one in each block) so you can control them with their respective handle.
