double chainring, single cog -- how?

Am thinking about how to convert a single speed hub/single chainring to double chainring setup (still single speed behind). Is there a common name for this?

How to do this, without using a derailleur? I have seen the Kore Chain reactor. Anyone tried using that? I can't really get a feel for its equivalent capacity terms, compared to a normal derailleur. It sounds like similar things might be common place for less common setups like tandems or recumbents.

Thanks for pointing me in the right direction,