I would also add to this that in order to open two instances of an app the app explicitly needs to support this. For example, you can't open 2 instances of Calculator.app side by side.
If only I could write it down in a text note and refer to it and many more, as opposed to keeping a calculator window open per prior calculation I want to refer to...
Or if only there was a tool like Soulver or Calca...
Yes you can write down the result of calculations and all relevant state and then clear the state of the calculator window before doing a new calculation. But why?
This is really annoying.