The exact technique depends on which records you wish to transfer.
I assume you are mainly interested in
Individuals and
Family groups.
The general idea is to use
File > Import/Export > Export > GEDCOM File and
Select who you want to transfer.
You then need to make decisions about linked
Source,
Place,
Media records, etc.
That exported
GEDCOM is then imported to the other
Project using the
File > Merge/Compare File command.
See how_to:exporting_gedcom_with_multimedia|> Exporting a Family Tree with/without Media that explains the options in much more detail.
If any of the assumptions made above are wrong, then please clarify what records you want to transfer.
However, it does beg the question why you want what will be duplicated records in two separate
Projects.
It implies that any future updates to those records, including adding new relatives, will have to be performed twice, or that export & merge process repeated each time.
Would it be better to merge the two
Projects in one composite
Project, which is what we usually advise.
See the recent
Advice about my own FT (16932) that discussed that very topic.