[Vuejs]-How to keep mounted in 2 component working same time in Vuejs?

I mean its wrong tactic.
MDN writes:

The onresize property of the GlobalEventHandlers interface is an
EventHandler that processes resize events.

https://developer.mozilla.org/en-US/docs/Web/API/GlobalEventHandlers/onresize

So when your second component has been mounted – window.onresize was rewritten

If you want to change something in a component when window be resized you can use "watch" methods https://v2.vuejs.org/v2/guide/computed.html

You need create a global variable like windowSize, when window was resized assign new size to windowSize, and in component watch windowsSize