The tranny strap is the only one you really need. The engine is attached by four large bolts and touches all round the circumference of the bell housing, so the changes of a bad connection to the engine itself is zero.
Likewise the generator/alternator - it's held in place on the stand by a wide strap which is bolted tight, and the stand is attached to the case by four blots, so the changes of a bad earth connection here are zero too.
So if the tranny strap is toast, replacing that is all you need to do. Don't just add another to the old strap though - you might end up having to bend the clamping piece at the end, or it might try to cock the mounting bolt to one side. Make sure the metal it clamps too is nice and clean. Whilst you are at it, check the battery earth strap where it clamps to the body. Sure you COULD add a second strap direct to the engine as described above, but that leads to the possibilityof some equipment still working off a "bad" strap (which can lead to poor working components and even electrolysis which will cause rust), so it's better to have one single (but very clean) path for the earthing throughout the vehicle - via the body itself.
Rob
Rob and Dave's aircooled VW pages
Repairs and maintenance for the home mechanic
http://www.vw-resource.com