Is it possible to merge two (or more models) in libgdx?
This issue has been solved. Libgdx library has some bugs which cause many problems with merging different models (while the same models can be merged without any problem). I found an apropriate solution to this problem and it allowed me to use VBO system without properly.
For more information please refer to those links:
slow model batch rendering in libGDX
http://www.badlogicgames.com/forum/viewtopic.php?f=11&t=10842