Adding Project

First, back everything up.

Then open the backups to make sure the backup worked OK.

Then, back everything up again, just in case.

Now, You can copy the activities in one project and paste them into the other, that’s teh simplest way.

Watch out for;

Activity code structure should be the same in both projects before you C+P. Failure to do this can lead to clipping or offsetting of the activity codes in teh pasted activities

Identical activity IDs will prompt a request for a new act ID for each activity copied. One way around this is to make sure that your activity IDs are only 8 characters long. Then, when you paste, select "add prefix" and put a 2 letter code that identifies the 2nd project.

Make sure you have no summary levels active when you paste, otherwise data that defines the summary level that you had selected when you paste will be added to all pasted activities

Best practice is make copies of the 2 projects then practice copying and pasting an activity from one to the other to see what happens and what the options are. For example, if you want both projects to be connected to a common phasing plan and you have built the same phasing plan ni each project, you can select to bring in "external links" at teh time of pasting. This will generate links outside the set of copied activities if you had links to activities in Project1 that do not have a corresponding activity in Project2, then a new blank activity will be generated (at least I think so)

A more advanced option is to work with export / import, but it sounds like copy and paste is what you want.

how are you Moutaz?

there are also merge and transfer functions in P3