Joining two furniture libraries isn't possible at this time. Please, fill a feature request as a reminder.
The ID isn't necessary, and the ID column won't probably appear by default in a future version. Only one piece of furniture with a certain ID may exist at a time in Sweet Home 3D. During program launch, if two pieces of furniture with the same ID are found only the first one will appear in furniture catalog.
I noticed that this is my 500th post :-)
You must really like the program! 
Thank you for your great support and for helping other users in this forum.
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Emmanuel Puybaret, Sweet Home 3D creator
