shorting the 9/12V AC at transformer?
You have not used proper switch symbols in your drawing but I guess you mean that you are considering wiring them as shown in Figure 1a. This will be a problem as there is a danger of short-circuiting the 9 VAC to the 12 VAC transformer tap.



simulate this circuit &ndash; Schematic created using CircuitLab
The SC5-S-DC5V relay appears to have a change-over contact so you can wire them as shown in Figure 1b. This prevents short-circuit of the transformer.