How to use ifelse to extract last word of a string in R?

I'm new to R and I could use some help.

I have two different datasets with people's names.

First database looks something like this:

    NAME            
JOSE SANTOS         
MIRIAM RIOS          
JULIANA SILVA
  

The second one looks like this:

         NAME                      
TIAGO MELO                         
JOSE FRAGOSO SANTOS               
JULIANA DOS SANTOS SILVA           
MARIANA RIOS 

Then I created a key using the first letter of the first name + the last name so I could merge the two datasets and see if the person from dataset1 is present in dataset2. It looks something like this:

  KEY               NAME_DB1              NAME_DB2
J SANTOS           JOSE SANTOS       JOSE FRAGOSO SANTOS
M RIOS             MIRIAM RIOS       MARIANA RIOS
J SILVA            JULIANA SILVA     JULIANA DOS SANTOS SILVA
T MELO                 NA            TIAGO MELO

Then I created another key that gets the first 2 letters from the first name + last name because M RIOS could stand for either Miriam Rios or Mariana Rios.

       KEY2               NAME_DB1              NAME_DB2
    JO SANTOS           JOSE SANTOS       JOSE FRAGOSO SANTOS
    MI RIOS             MIRIAM RIOS                NA
    JU SILVA            JULIANA SILVA     JULIANA DOS SANTOS SILVA
    TI MELO                 NA            TIAGO MELO
    MA RIOS                 NA            MARIANA RIOS

What I need to do now is merge those 2 datasets by these 2 keys. The logic I need to follow is:

if LAST NAME == "RIOS", then it uses the second key (2 letters + last name). If LAST NAME is different from "RIOS", then i'll use the first key (1st letter + last name).

I don't know how to do that. I can't use the second key (two letters + last name) for everything because some names of my datasets are like this: R D RODRIGUES, for example.