Building an appliance with two plugs to get double the voltage?
if you plug in a toaster that uses 60 of the 110 volts then there is only 50 left.


No, the devices are wired in parallel, which means all devices in a home share the same voltage.

Things are not wired in series like this:

110 V ---------HiFi--------Fridge-----.
                                      |
neutral-------------------------------'


which might mean the HiFi gets 85V and the Fridge gets 25V and neither works because each needs 110V to work.

But are wired in parallel like this:

 110 V   ----------+--------------+
                   |              |
 neutral -------+--------------+  |
                |  |           |  |
                |  |           |  |
                HiFi          Fridge


It may help to think of the usual water analogy. If you have a very large tall tank of water, the pressure at the bottom only depends on the height of water in the tank, nothing else. It doesn't matter how many pipes are connected at the bottom of the tank, the pressure in each pipe is the same. You can't double the pressure by connecting pipes together creatively.

You can double the flow obviously, if one pipe can deliver n gallons a minute, two pipes (the same size) can deliver 2xn gallons a minute. But flow is like electric-current (amps) not like electric-potential (volts).

Your house does have a limit on the total current that can be supplied. This is because wires to your home would get hot if they carried more than their rated current. It is a bad thing when wires get hot, melt or set fire to the surrounding buildings and to the people in them.

So if your house has a 200A supply and charging your Prius takes 100A that only leaves 100A for the rest of your appliances. But that is current, not voltage.


  why (if) it is not possible to use two plugs to get double the voltage?


Because you'd just end up doubling the wires (to no effect) or shorting 100V to neutral and blowing a fuse / tripping a circuit breaker.

You can do this with multiple batteries. For example, connecting two 1.5A AA batteries in series (one after another) does create 3V. But you can't create 3V from a single 1.5V battery just by creative wiring. Your household 110V AC supply is more like a single battery with lots of items connected by wires.

The USA does use a "split-phase" supply to houses (think two-batteries) which means you can create 240V by making use of both phases. There are rules about how you do this and anyone without a basic grasp of the principles of electricity should not mess around with this.