You can bounce!
You can't sync them together, but you can fill up four tracks on one... (Guitar, bass, Keys and Drums) and then transfer to the second deck in stereo and fill up the other two tracks with vocals and lead guitars etc.
This has two advantages:
1. You will still have the original 4 tracks on a separate tape to work with later.
2. It will force you to think about arrangement and mixdowns in advance. You will become a better musician.
*It is my humble oppinion that fewer tracks = a challenge to play better. In the old days they would have to all get it right in one take or the whole 16" acetate disk was trashed.
Now we have so many tracks and virtual takes to rely on that we may become lazy, knowing that we can edit/cut and paste the song together.
Dom Franco