I am always having trouble importing and reusing rigged characters with bones and skeletons into other scenes so I tried to come up with a solution that so far worked for me so I decided to share it so that others can use and maybe improve it.
First thing you have to do is install the plug-in that will import the characters into your scene. I tweaked the original script that was used for the import of simple skeletons so that it works for rigged characters with bones. Dowload the add-character.rar file and extract it and copy the folder add-character into the plug-in folder of Synfig. On Windows that folder can be found in …Synfig/share/synfig/plugins. The add-character folder contains a python-file that is the actual plug-in, an uncompressed sif-file character.sif that is the actual file that will contain the rigged character you want to import (for now it only contains a dummy character) and an xml-file that holds the name of the plug-in and that points to the python-file. After you copied the plug-in in the right place it should appear in Synfig under Plug-Ins.
Next you want to make and rig the first character you want to import. Just make a character and rigg it with the skeleton layer and add some bones to it. Make sure that when you name your groups that you set (char) in front of it. So a group called Head will become (char) Head. A group called body will become (char) Body, Skeleton will become (char) Skeleton, etc. Now give your character a name and save it as a compressed sifz-file in a place where you usually save your characters.
Next you want to make your second charater and rigg it. Again use the (char) variable in front of every group. Give the character a name and save it.
Now open up a new file that will contain the scene that you want to compose. Leave the scene-file open and open the first character-file that you made. Save the first character as an uncompressed sif-file into the add-character plug-in folder and name it character.sif. Close the character-file and go back to the scene-file and under Plug-Ins click on Add character. The plug-in will import the first character and will add a number to each of the (char) variables depending on how much characters you already imported. Leave the scene-file open and open the second character that you created and also save that as an uncompressed sif-file into the same add-character plug-in folder again under the same name character.sif. That will overwrite the first character but since you also have a backup sifz-file of it that does not matter. Close the file of the second character and go back to your scene-file that was left open and under Plug-Ins click Add character again. The second character will import and the (char) variable will be set to two so that it is easy to spot the two imported characters in the layers panel.
You can now save the scene and start animating. If you want to change or edit either of the characters and or their skeletons I suggest that you open the original sifz-files of the characters and import them back into the scene after the changes are being made. I know this is a long post but it helped me out in more then one way. I will add the plug-in as a rar file and also include the final scene-file.
Link to youtube movie showing the basic steps: https://youtu.be/3nEoO_6wtfs
scene01.sifz (12.3 KB)
add-character.rar (4.15 KB)