I see, well. I remember opcodeexe making a mod where you can run multiple instances of SA-MP at the same time, but that only works on 0.3.7. I can't remember any other solutions to this sadly.

[Video: https://www.youtube.com/watch?v=_HnNub36mvY]

Edit: I looked into it a bit more and ThirteenAG's Widescreen fix contains an option to allow this.
https://github.com/ThirteenAG/WidescreenFixesPack/releases/tag/gtasa

After installing you need to navigate to the "scripts/" folder and open "GTASA.WidescreenFix.ini". You need to edit "AllowAltTabbingWithoutPausing = 0" to "AllowAltTabbingWithoutPausing = 1".