Is there any proble to install IJCAD on the PC which AutoCAD is already installed ?

IJCAD is the individual software with AutoCAD.

You can use IJCAD and AutoCAD together on the same PC.  

The standard file format of both IJCAD and AutoCAD is DWG format. If you associate DWG file to IJCAD, when you double click the DWG file in the File Explorer, IJCAD will automatically execute.

This association can be changed.

Reference :

• To change the association of DWG to AutoCAD
• To release the association of DWG file