This is a known issue, appears to be purposely implemented by jagex.
 
	Save this code into a notepad and then save as a .bat file (name it whatever you like as long as you put .bat at the end)
 
	 
 
	 
 
	@echo off 
	  
	  
	:GAMETYPE_SETUP 
	SET /P gametype="Gametype (rs3/osrs): " 
	IF /I "%gametype%" == "rs3" GOTO AMOUNT_SETUP 
	IF /I "%gametype%" == "osrs" GOTO AMOUNT_SETUP 
	GOTO INVALID_GAMETYPE 
	  
	  
	:AMOUNT_SETUP 
	SET /P input="Amount of clients: " 
	SET /A amount="%input%"*1 
	IF %amount% GTR 0 GOTO RUN 
	GOTO INVALID_AMOUNT 
	  
	  
	:INVALID_GAMETYPE 
	ECHO Invalid input 
	GOTO GAMETYPE_SETUP 
	  
	:INVALID_AMOUNT 
	ECHO Invalid input 
	GOTO AMOUNT_SETUP 
	  
	  
	:RUN 
	FOR /L %%i IN (1, 1, %amount%) do ( 
	    IF /I "%gametype%" == "rs3" ( 
	        START jagex-jav://runescape.com/jav_config.ws 
	    ) else ( 
	        START jagex-jav://oldschool1.runescape.com/jav_config.ws 
	    ) 
	)
 
	 
 
	 
 
	Once you've done that, double click it, a command prompt will pop up type "osrs" and then type the amount of clients you want to open
 
	 
 
	EDIT: credit to a user "Savior" on another website that cant be mentioned