According to
the Wiki, there are no training dummies at all in Chapter 2, but there are two of them in the Temerian Quarter, and two of them in the Order's outpost, in Chapter 3. Unless you sided with Siegfried in TW1, I don't think you'll be able to enter either area safely if you've gone down the Iorveth path in Chapter 2.
If you enter either camp - and they're hostile - it doesn't mean an instant death, but the Order will infinitely respawn, and if you kill more than one guard in the Temerian camp, I think you'll be thrown in jail; so the latter might be an idea. Once you're ready to progress with the story there, rush in and try to get the training dummies as quickly as possible, and then let the guards capture you if you can't make it back out in one piece.