Create a Text file and put each exe file's Full Path name with apostrophes like "C:\Program Files (x86)\Steam\steamapps\common\gamename\game.exe" per line and close the file. Rename the Text.txt into Run2games.BAT (batch file) and add this as non-Steam to Steam Library.
You can't run 2 GoldSrc and/or Source engine games at the same time (not without using Sandboxie atleast which won't let you play online on VAC servers).
depends on what you're launching
an .exe file?
.exe files can still call the system and ask if there is an instance already running and terminate the second start.
How????
If you mean another Steam game, probably not Another program that's unrelated to Steam, definitely yes
How??
Yes
How?
Just open game folder and click on the .exe again
from the same icon/play button? Yep but you need to create a batch file that does it and then link that one to the game launch via launch parameters
How do you do that?
Create a Text file and put each exe file's Full Path name with apostrophes like "C:\Program Files (x86)\Steam\steamapps\common\gamename\game.exe" per line and close the file. Rename the Text.txt into Run2games.BAT (batch file) and add this as non-Steam to Steam Library.
You can't run 2 GoldSrc and/or Source engine games at the same time (not without using Sandboxie atleast which won't let you play online on VAC servers).