Using two small identical bookshelves for 2 channels

Doubling the speakers is only a 3dB increase. If you actually need more sound than a single (pair) speaker can provide, you should move to a radically larger and more efficient drivers. You must live in the countryside where you can blast high volume without offending the neighbors? Also, 4" drivers cannot produce much bass without distortion caused by the large cone excursion. If you are size constrained, consider a sub like computer 2.1 systems.