Actually, when grouped with two toons, even with group bonus you get about half the xp per kill you would get solo. I've leveled alot of toons solo and duo and have observed this.

So to answer the OP's question, if you want the highest speed level the shammy solo as enhancement. If you want *TWO* level 80's as fast as possible dual-box them. It'll take about 33% longer than soloing but in the end you'll have two 80's in alot less time than it would take you to level each one solo, separately.