Formula Fields

I need to have 2 fields that calculate the display, based on a look up (I think)

I have 2 diffent ID fields that link to the same 'look up' fields. These field can be null so I wanted to do something like this but I can not get the correct syntax


If isnull(Field1) then 'Blank'
Else Select Fname, Lname from Table2 where table2.Field1 = Table1.field1

What I had and it appeared to work, but I have a hard time believing it is correct is;

If isnull({Field2) then 'Blank'
Else Trim({Fname}) & ' ' & Trim({LName})