I always assumed that the "I need a second front" thing was just a mutual understanding between the two-- an excuse for Angel to not be there.
Buffy didn't want Angel to be there; whether it be because she would be too distracted, or because it "wasn't his fight" to fight, or maybe a more personal reason in concerns for his well-being... it was clear that she didn't like the idea of him sticking around to fight.
To me, it always seemed as though she scrambled for an excuse for him to leave town, and that's what she came up with. She knew that wasn't why she didn't want him there, he knew that wasn't why she didn't want him there, but it worked. They both understood the real reason (whatever it may be-- that's up for debate), and were fine to just leave it the way that they did. I doubt that there were really any intended preparations on Angel's front, in reality. Again, just how I interpreted it! 🙂
But in answer to your question, I would probably gear up. Have an eye on Sunnydale, someone stationed to see it coming and everyone else just waiting for it to come. Grab anyone and everyone that you could, form a makeshift gameplan and get lots of weapons. Dealing with the first, it'd be all I would think to do. You can't defeat the first itself; just its army. And if the gang in Sunnydale couldn't beat that army, I think going at them full force is I would think of trying!