Sometimes when attempting to warp multiple clips, you’ll see this in the clip view:
This is telling you is the main caveat of warping multiple clips simultaneously: they all have to be exactly the same length.
The solution is (not surprisingly) to make both clips the same length. This is easy to do, provided that the difference in length is at the end.
In other words, if there’s some extra silence at the end of one of the files, or the vocal track cuts off before the big drum solo at the end of the song, you’re fine. However, if the vocal track starts right at the beginning of the first verse, but there should some empty space at the beginning to account for an instrumental intro, you’ve got a whole other problem to solve. I’m skipping that problem for now.
To get started, drag both clips into the Arrangement view, lining them up so they both start at exactly the same time:
Now, hold down Shift to select both clips…
…and use the Consolidate command from the Edit menu.
This will cause Live to write two new files of exactly the same length. (You may try to get clever and only consolidate the shorter file so it matches the length of the longer one. Good luck. It usually doesn’t work.)
Voila! Warp away.