That kind of a tough question.
Usually, situations like these demand settling on the lowest common denominator. In your case, the lowest common denominator would probably NOT be resolution-based -- as that is an easier variable to control and convert. In your case, I would assume that the interlace/progressive profile would be the driver your workflow decision.
First of all, you need to consider the capabilities of your NLE. If you're using something like an Avid Media Composer, you can use the built-in motion adapters to convert any resolution/framerate -- to any other resolution/framerate in realtime (with no rendering).
For example, in Media Composer, you would import your GH13 720p/50 media into one "project" -- and your GH2 1080i/50 media into another "project". Then you would open your 720 media -- while you were in your 1080 project -- and it would automatically be converted to the 1080i/50 profile.
That would give your the 50 i/p time domain that is required for smooth action.
At that point, you would just edit normally and output your final sequence for DVD and Web compression. However, this type of mixed media will require a complete workflow test (beginning to end) to make sure your deliverables (DVD & Web) are of acceptable quality.
Hope that helps.
Btw: This advice is based on my assumption that the GH2 does NOT do 720p/50 -- NOR 1080p/50
Thanks. My NLE is Edius 5.5 at the moment, soon 6.0.
It will take all framerates and formats.
And yes, the GH2 does produce 720/50p. But not 1080/50p