I have the exact VPI you are referring to and I see two ways to "improve" the sound.
I don't like the fact VPI arm has to go thru many connections before reaching phono stage. I will be building a cable myslef with female Lemo on one end and male RCA on the other. simple to construct, doesn't cost much to build, should be a good upgrade in sound.
as to adding a 2nd arm, as long as your shelf is large enough you can "place" a 2nd arm on the back sitting in a heavy pod of some kind. German table Musical Life is built that way, the arm simply sits in a pod next to the table. another option is to build an arm board that's attached to the left rear feet or bearing nut.
if there are enough interest out there, we probably can get machine shop to build some at a discount.