How would you like it to know where the last name begins?
Sometimes a name is not as simple as "John Smith", it may have several spaces such as "Stephane da Costa", or "Billy Bob Thornton"..
If you just want it to break on the first space; you can use a 1 line script in OnSetText for the text object:
Text = Replace(Text, " ", " {LastName}",1)
You won't see this applied in layout, but it should work fine when putting a scene online from Sequencer mode.
#XPression