To achieve your goal you need to use the IF function in Excel with multiple conditions
=IF(AND(ISBLANK(DAName1), ISBLANK(DAName2)), ", IF(ISBLANK(DAName2), DAName1 & " has been added to your policy as a delegated authority and can now access your information and make claims from your name", DAName1&" and "&DAName2&" have been added to your policy as delegated authority and they now have access to your information and can make claims on your behalf"))
This formula works like this:

Checks if both (DAName1) and (DAName2) are empty. If both are empty, an empty string is returned.
If (DAName1) contains data and (DAName2) does not, a message is returned indicating that (DAName1) has been added.
If both (DAName1) and (DAName2) contain data, a message is returned indicating that both have been added.
Hope this helps!