Strange is the WRONG person to give any kind of prep. And now you're giving him 2 days of it? He can summon demons within near minutes, but now you want to give him 2 days? He will summon all the demons of Hell, the Dark Dimension, Cyttorak's realm, Watoombs realm, Hrggorth's, Denak's, Oshtur's etc.
It will look something like this...
http://img140.imageshack.us/img140/3803/strangearmy28pl.jpg
http://img140.imageshack.us/img140/1361/strangearmy11dn.jpg
........but this time with thousands upon thousands of demons, imbued with strong magic.
Not only will he have an Army of demons at his side, he'll also have the Eye of Agomotto. Using that he would BFR the entire JSA, and only a handful of them will be able to escape, making the fight very lopsided indeed. Not only will he have the All-Seeing-Eye, but he will have the Book of Vishanti, putting him on par with the very beings from which this book is named. If you thought that wasn't enough, he has to power to steal powers from the most powerful entities. He can steal powers from Cosmic Beings, and assume their role. He will go to the likes of In-Betweener and explain to him the dire situation, and that he needs his powers for a time, or he could go to Shuma-Gorath's realm and steal his powers like he has done before, adding further to his omnipotence. He has spells that can kill with the simplest of gestures, that he never uses. If JSA poses a big enough threat, I'm sure he will employ those gestures.
That's just Dr. Strange alone. Thanos is a whole nother problem entirely. He'll probably leave Strange and the legion of Demons to do most of the dirty work. Oh, he'd just traverse through time and obtain the Cosmic Cube (like he's done on a number of occassions), and become nigh-omnipotent like Strange. Nothing special. Just two nigh-omnipotent beings with arguably the most experience in comics. Big whoop. Or if he doesn't want to labor too much, he can just fix his ship with a ray powerful enough to kill most of them. But he'd probably rather have fun with the cosmic cube.
Archenemies job is to take notes.
Thanos and Strange most likely win this one with prep.