There’s no harm adding Z2 of you can manage it. If you look at the LDMV Tri plans you’ll see plenty of VO2 workouts with that structure (eg Mist which is basically Spencer +2 with about 20 mins of Z2 added onto either end).
Though obviously not something we amateurs might want to replicate, I’ve seen a good few Pro workouts were they drop their VO2 workout into the middle of a 4 hour ride. If you could manage that then fair play to you 
- the only thing that really matters is not compromising the goal of the session.
Similar to @ojtCycling I find the extra run in gives the legs a better chance to warm up - Mist is easier for me than Spencer +2.