Cross game interaction with trials frontier is most likely around completing he same track of both games. On frontier I unlocked a computer thing in front of the saloon which requires me to get a gold on three tracks, on both frontier and fusion, to unlock items in the game (just appears to be rider gear so far). So there must be some shared tracks at least.
I hope the tricks and 'do x number of backflips' etc are separate for the normal races. On frontier they slow you down, but sometimes you have to beat a ghost time *and* do 5 backflips etc, and I'm not a fan of those combinations. Give me stunts or a time limit, not both.
Still in for day one on PS4, most likely digital and full price for later tracks.
Oh and for those that hate UPlay, almost certainly the console versions will want you to sign in to Uplay - I guess that is the only way they can connect you up with trials frontier for instance.