That's the opposite of what I said. If the quiver were X-shaped and contain bolts long enough that they cross one another, then this quiver would have to be one-on-top of the other, probably in an awkward position. Otherwise, arrows will get snagged on one another.
What I stated was that since this wristbow would likely not have long arrows, why not have someone who dual wields one have two quivers side-by-side behind opposite shoulders? Presumably, the arrows would be short enough.
Given how wielding a weapon in the same hand as a buckler provides a penalty to hit, so should the wristbow.