I can tell you pu$$ies don't "go bush" very often!
1. A tarp between two landies is a balls, mainly because the wind just HOWLS under them due to their SUPERIOR clearance. Plan to block the underside of the upwind vehicle* if you do persist with this wrongheaded notion
2. Nothing to attach to - lol. Mariners is it? Fix the tarp to a batten, fix lines to the batten and heave lines over the top of the sloneranger, fixing them to chassis sidemembers. You can also secure to the back door hinges this way
EDIT * A Terranopin is probably ideal for this application.