The problem is the filename which is in both cases the same: package_seeeduino_boards_index.json.
This file will be installed when you’re using a portable installation like me in your “/portable” map beside the other json-files from added packages like “package_adafruit_index.json”. (I think when you’re using a non-portable install the Jsons are placed in the “/hardware” directory) When you use the link on seeedstudio.com that json will be installed and when you use the link on githubusercontent.com that json will be installed and the last one overwrites the other one.
I did self make a workaround and downloaded manually the SAMD json, renamed it “package_seeeduino_SAMboards_index.json” and placed it in the directory with the package json-files.
I have only the link for the AVR in my preferences URL list copied. The SAMD link is NOT pasted in the preferences URL list.
After this modification I can install AVR and SAMD from within the board-manager and I can use both platforms but…not everything is ok because I get a load of error-messages about many tools which are not installed as supposed…but most of the time the sketches work as they should.
A good solution would be one combined JSON from Seeed where both platfroms are integrated…or two json’s with different names so the SAMD json will not overwrite the AVR json.
As a great fan of Seeed products I think this modification should be not a big challenge for the developers at Seeed and it will help a lot when you will use both Seeed platforms.
This moment it is not advisable (especially for new users) to buy and use both platforms from Seeed.
(which is not good because I want to share my enthousiasm about both Seeed platforms with others )