If you come directly from the battery post with your seperate ground wire, you better fuse it.

Here's what can happen:

1. Bad ground connection from battery to motor.
2. Good ground connection to 2-way radio.
3. Because it is a better connection from battery ground, to 2-way, to the place where the grounded 2-way mounts and ground of antenna, to frame, to motor, to starter; that's the path the current takes.
4. Circuit board traces are not up to the task of handling starting currents, so they make green smoke.

If you're absolutely, positively sure that the equipment running off the seperate ground has no connection to anything that's grounded, then you don't need a fuse. Otherwise, it's asking for trouble and expense.