As others say, two helmets, the changeable helmets don’t fill me with confidence like a full facer does, and i dare say they’ll be heavier and less comfortable than a normal helmet. You could always go middle ground and get a dropframe for less adventurous days, but it’ll get expensive having a a couple of £200 helmets and your normal one!
Personally i just wear the full facer for uplift days, DH only days and stuff like that, where i can go slow on the ups and have the confidence on the downs, if i was doing full loops of several km’s then i’d probably leave it in the car and use a normal lid, it’s all about the risk on the day, but reality is that full facers aren’t all that bad for some miles, i just don’t enjoy the lack of peripheral vision for so long, that gets me more than heat or weight.