I thought this would take half an hour ...
So I did the calculation and added enough wire to the 10m dipole (which is below my 17m dipole) so I should have been able to trim back and get a good swr on 15m. But it didn't happen. I never got anything close to a dip near 1:1, and my swr was also affected on 17m. I was using my Comet ant. analyzer and while trimming I went past where it should have been correct. I thought I wasn't paying attention so I actually started over with a new piece of wire, but the same thing happened.
After two hours of this I finally decided to keep trimming until I got to the next band (12m). As I cut more and more wire, I got better and better dips as the resonant freq. went up, and the 17m dipole started acting better too. Finally, I got a very nice 1:1 swr at 24.960 MHz, and my 17m dipole was back to normal.
So I don't really know what was going on, but I suspect that it had something to do with the geometry of the two dipoles and some sort of interaction between 15m frequencies and the chain link fence below both dipoles. Perhaps this particular ensemble of physical properties and the fact that 17m and 15m are "close" that I just couldn't achieve the desired results.
In any case - hello 12 meters!