Thanks for responding, but that doesn't seem to work. I see no "Multi-Camera>Enable" when right-clicking on this "second sequence," and furthermore I need more than one camera in my new multi-camera sequence, so right-clicking a single sequence doesn't seem relevant. Maybe there's something I'm not understanding about your suggestion?
Thanks again for your help. So that "enabling" worked, but it brought me to another halt, which may be a different issue: After telling Premiere to make a multicam source sequence with sychronized in-points (which I set in each of three camera sequences), the sync failed.
So I'm still failing to use sequences for a multicam source.
1. Create a sequence matching the media.
2. Add your clips to that sequence, each camera on it's own track.
3. Sync those clips manually.
4. Create a second sequence with the same settings as the first.
5. Nest the first sequence into the second.
6. Right click that nested sequence and select Multi-Camera>Enable.