Should I get shocked? If not, why?
Let's draw the schematics for a few possible scenarios. The ammeter in the schematics represents your body and its resistance.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1.


  Let say I touch the positive terminal of 400 V circuit and the negative terminal of 12 V circuit. Should I get shocked ...?


It depends.


In Figure 1a both circuits are floating with respect to ground and to each other. In theory no current can flow from one to the other.
In Figure 1b one circuit is grounded but the other isn't. Touching the 400 V grounded circuit would be a really bad idea as any path to earth through your body could cause electrocution. If you were perfectly insulated then no current would flow because there is no return path to the 400 V negative.
In Figure 1c both circuits are grounded so touching +400 V and the negative of the 12 V is, effectively, touching both terminals of the 400 V supply. Electrocution is likely.


All of the examples are using DC. When AC is involved you have to take capacitive coupling between each circuit and ground and between the human body and ground. This can be high enough to allow dangerous current to flow.