First, nested tokens are not possible in Marketo: Re: Is it possible to use a token within a token?
Second, it looks like those are just Tokens and not My Tokens, which actually means we can go about this a different way.
Correct me if I'm wrong, but it appears that you would prefer to display someone's secondary name, and if they don't have a secondary name, to default to their first name, and if they don't have a first name, default to "Valued Customer".
This is a bit of a workaround, but you could create a new field called "Greeting". Then, in your program, run a Smart Campaign whose flow is "Change Data Value: If Secondary Name IS EMPTY, {{lead.First Name}}"
Back in your email, instead of writing {{lead.Secondary Name:default={{lead.First Name:default=Valued Customer}}}}, you would just write {{lead.Greeting:default=Valued Customer}}. This will populate the greeting token with either the secondary name, first name, or Valued Customer depending on what applies to their record.
Hope that makes sense!