IF statement to join 2 first names only

IF statement to join 2 first names only

I have set up on a contact layout to have a partner/spouse with secondary first and last name fields. 
I'm trying to set up a formula field where if the secondary name is empty, the only the first name will appear in the 'letter' formula field, but if there is a secondary name then the formula should conjoin the two names together as "name and name2". 

I've tried to do this as both boolean and string, but each time it gives a bracket error but I cannot find the error to resolve. 

I am unsure if I should be using ==null, or empty, or something else for the IF statement to check if the text field in question is empty or not. If I use just the concat part of the code I receive no errors, it is only when trying to include it as part of an IF statement.

The code I am using: 
  1. if(${Contacts.Secondary First Name}==null,${Contacts.First Name},concat(${Contacts.First Name},' and ',${Contacts.Secondary First Name}))
Any help or ideas greatly appreciated!!