If you have a Power Cab AND an SB5 you should not be using a toggle switch for anything.
I do not recommend you use them for these reasons:
1. human error - sooner or later you WILL forget which way the toggle switch is thrown and inadvertently make a programming error or think you have equipment that is not working correctly when in fact you forgot to throw the switch one way or the other. I have seen it happen over and over and over.
2. the switch you install can (and will fail) eventually. It is a mechanical device. It can also introduce noise into the digital signal causing programming errors.
Since you already have a Power Cab AND and SB5, you should be using the Power Cab separately in the original stand alone configuration with the PCP and the flat cable. This is where you setup your programming track and program your locomotives using the programming track MODE of the Power Cab.
However, most people do not need to use programming track mode unless they need to READ the CV values of the locomotive.
for most programming needs like changing the locomotive number from the factory default of 3 to a road number you can and should be using the OPS mode programming / (POM) programming on the main of the SB5 booster.
The SB5 does not have a program track mode, or a program track output. You can however do programming on the main. The SB5 is designed as the second purchase / upgrade after the initial Power Cab purchase.
The normal / recommended setup is as follows:
Part 1. - The SB5 and the power supply provided with it are connected to your main layout full time, and maybe a UTP panel or two connected via the cab bus jack. You use the Power Cab or any other NCE cab connected to a panel. Trains with remain running when the Power Cab is unplugged since the SB5 is now powering the layout. The Powercab is now being used as a regular cab / dumb terminal. You use the coiled cable with the SB5 since the Power Cab is being used as just a cab. This is also known as using the Power Cab in "Pro Cab Mode"
Part 2. – On your workbench you use the Power Cab as a programming and testing workstation. This is where you use the original configuration with the Power Cab, the Power Cab Panel (PCP), flat cable and the original P114 2amp power supply. The Power Cab gets plugged in on the left side with the flat cable with the LED below the jacks.
The Power Cab will be used in both locations but in different ways, all you have to do is unplug it from one location and plug it into the other. So now you have two completely separate areas that do different things just by unplugging / moving the Power Cab around. You can also get ANY other NCE cab to run trains on the big layout, or even go wireless.
A programming track mode and a program track are essential for the ability to READBACK a CV. Read back is not available on the main using POM (Programming on the main) also known as OPS (operations) mode programming.
Some decoders have limitations on what items can be changed in POM mode and / or OPS mode.
The best solution for you is to setup a dedicated test track / workbench track with the Power Cab and the Power Cab Panel (PCP). Add the USB interface to this for programming via a PC / JMRI. Add the SB5 smart booster/command station to run the layout. The Power Cab will function as a cab when used on the main layout and as a command station / programmer on the test track. Once you have the SB5 in place, you can also upgrade to wireless cabs later if you want.