I would like to combine two Samaccountnames into a single user for uba timeline events.

my example is a normal user account and the admin account. These two accounts are for the same user in the same domain but two separate AD user accounts. Is this possible with coalescing or a manual add?

Yes, you can manually create a csv file with the field names as the header and the unique name from both accounts as comma separated values. You can then take the csv and import it into UBA. After coalescing is ran those two users should be grouped into one.