It doesn't work out to twice the maintenance. In the Volt, then gas engine is only used when you need to take a longer than normal trip. Even then, it is mainly a series generator that runs at a leisurely rev range to supply power to the EV system. It is not stressed the way a normal engine is.

Maintenance schedules for the engine are extended as a result. Oil changes, for instance, are automatically tracked and I only had to do one oil change after about 2 years.

I'm coming up on 3 years with a gen 2 Volt and it's been very reliable and more fun to drive than the ICE vehicle that it replaced.