I just want to know, We can combine model in Caesar II ?
I want to combine 2 projects for storage as 1 file.

Thank you.

Yes. There is an icon in the Piping Input to "Include Piping Files". If you want to permanently include the 2nd model, make sure you use the "Y"es option.

thank you again.

I get an error of non compatible unit files when I try to combine 2 models that were done using 2 different unit files.
Any idea how to solve this?

You need to have both files with the same units therefore convert the files in order to have the same unit file.
In Utilities tab you have the Change Model Units command.