Holdsworth is more famous (I don't think the term really applies here);) because he is a real innovator BOTH in a technical level AND in a songwriting/harmony/conceptualization level.
When you hear Jordan's CDs, they are 80% covers, and some of them kind of suck, some are only good covers and a couple are great (his Ravel's Bolero is great) but, when you think wow, that guy is alone it's quite amazing. While Allan plays mostly his tunes, and when he covers something (the only cover I heard was the Beatles Michelle) he twists it in such a way that you can only bow and say you're a master.
Check out Charlie Hunter, who plays both Bass and guitar parts in his duo (which sounds like a trio...).