both point to the same Discogs entry and all the data is the same (barcode, catalog number) except the country of release.
The difference is that the first one missed the Rehearsal Tapes (video outtakes of studio recordings and interviews). I have the second but I am pretty sure the entries should be exactly the same. I cannot find any information about a separate release date. Even if the European release would be available in different countries on a different date, does that warrant a whole new entry when everything is exactly the same?
Read the original edit histories. See if any reason given there for the differences.
One thing that will take a little time is you’ll need to adjust both DVDs to have the same track list before you can make the merge.
Keep the separate UK and German dates as it is common to have slightly different dates across Europe. In 2008 Germany tended to release on a Friday, and UK was a Monday which fits perfectly with these two dates. And unlike Discogs MB allows multiple release dates like that (i.e. don’t just replace it with a Europe)
If the artwork changed across Europe, or these had different manufacturers, then it would be separate releases. Don’t think that is the case here.
At a quick glance I can’t see any reason not to merge these.