Oil cooler line question

[vilant]

Have a couple questions if someone doesn't mind helping out. Finally got around to do some maintenance and I finally discovered the cause of my mystery oil leak. Turns out oil is slowly leaking out of the bottom of one of the 2 oil cooler lines directly behind and a little below the oil filter. The way it ran down to the floor kept misleading me. So my questions are;
What is more common the nuts coming loose or the crimp going bad (can't tell which part is leaking)?
If I need to replace the lines, do I need 2 wrenches? One to hold the nut closer to the engine and spin the other. Or do I just spin the one and both come off? See pic below, arrows point to the nuts I'm talking about.

What size are these? They seem pretty damn big, larger then any wrench I have.
And finally, are these lines full of oil or just residual after engine is off for a while?