I have seen people tying seven people to one plane with the grappling hook on YouTube. I don't know their system platform, but is this possible on PS3 consoles? It would be extremely useful for me to grapple more than two things together, so help me out.
Grappling in Just Cause 2 is only possible between one object and another.
If you've seen any videos with extraordinary grapples or say....flying, then they're likely using MODS for the PC version.
You can view and download them here: http://www.justcause2mods.com/
There are no MODS for the PS3 or 360 versions to my knowledge.