Can you have a TL on one ship, but use Deadeye to fire a totpedo at another ship using the focus token?

Yes, deadeye lets you use the focus instead of the target lock. If you have a target lock and a focus token you can choose which one to spend.

If you choose to use the target lock to fire the torpedo, it MUST be on the ship with the red target lock token on it. If you choose to use the focus token to fire the torpedo, it can be against any target within your firing arc, and does not have to be the target locked ship. Of course, if it isn't the target locked ship, you can't spend to target lock to modify the attack dice on that attack.