Yes, vertical is much better, but you should know how complicated room acoustics get with additional drivers not built to coordinate with one another. You’re introducing a lot of extra energy into the room.
Imagine for a moment that you are trying to spray a garden hose set on "conical" onto a specific spot on your garden. Then, you put another hose just like it next to it -- or atop it. Just think about the kind of water sprays that would result at the intended target. You’d get a lot more water, for sure, but consider the many ways the water sprays would miss the target -- or, more important collide with itself along the way. On a hot day, on a human body or set of plants, none of this would matter very much, but for human ears which are trying to create precise sonic images -- in stereo, no less -- this will create quite a mess.
Imagine for a moment that you are trying to spray a garden hose set on "conical" onto a specific spot on your garden. Then, you put another hose just like it next to it -- or atop it. Just think about the kind of water sprays that would result at the intended target. You’d get a lot more water, for sure, but consider the many ways the water sprays would miss the target -- or, more important collide with itself along the way. On a hot day, on a human body or set of plants, none of this would matter very much, but for human ears which are trying to create precise sonic images -- in stereo, no less -- this will create quite a mess.