install abyss, turn multiclient to 1
That'd certainly let you launch multiple clients - but Steam's multi-client detection is not equivalent to Mabi's multi-client detection. Steam looks for a "Client.exe" executable that is launched in a similar fashion to it's Mabinogi's own "Client.exe" executable.
You could try to rename Client.exe to a different name for MabiPro (Client2.exe) and then use a direct launch with a shortcut, as well as using Abyss to set multiclient ability to true.