While you’re waiting for something official to support meta-goals…
This particular case is possible now, with the exception that you have to set up the auto-synch outside of Beeminder. Either IFTTT or Zapier can do that.
The >0 case is possible within Beeminder, if that second goal has a custom aggregation method of nonzero
Fancier cases are certainly possible within Zapier, or possibly in the new multi-step IFTTT or in the old IFTTT with creative use of our Macros