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