I don't think anyone is clear on what exactly your question is.
Q1) "What would be the design to setup s2s vpn for two different sites?"
- Define design.
- The wording "two different sites" is implied already in the statement "s2s vpn" or do you mean something else all together?
Q2) "Would if fall back in case first site goes down and comes back up?"
- Define fall back.
- Define "goes down and comes back up"
- if WHAT specifically goes down and comes back up?