All the most interesting roles I can think of are about interactions between aspects of two spheres. What would be the best way to go about this, pick one?

Just pick one.

The single-sphere is mostly for the benefit of solidifying a concept for those who struggled to come up with a cohesive role.

Your own character can be as multidimensional as you'd like. Most of Amaranthe's follows, for example, have a role inclusive of the interactions between Seasons, Fertility, and Death (by the time they are tattooed, even if not initially.) But usually one has some prominence, or greater initial prominence.

There are also many role themes that may not be clearly defined by an existing sphere. In particular I feel we're missing some opposition spheres to existing ones; we have Order to counter Chaos and Dedication to counter Sloth, but no Humility to Pride, Cowardice to Courage, Ignorance to Knowledge, Charity to Greed, etc.

But you just make do with what's available. As long as your RP doesn't *contradict* to your sphere, no one will care.

In general, you can have as many beliefs in different spheres as you wish although one has to be central when concerning the character due to game mechanics.

For example, in all of the religions that I've followed, each sphere the god represents should be embodied by the follower even if, as you say, only one is the chosen sphere.

Just do your best to incorporate the look and feel of the character that you wish to embody those sphere aspects, that you can, and enjoy playing.