  We calculate optical conductivity for bilayer dice lattices in commensurate
vertically aligned stackings. The interband optical conductivity reveals a rich
activation behavior unique for each of the four stackings. We found that the
intermediate energy band, which corresponds to the flat band of a single-layer
dice lattice, plays a different role for different stackings. The interband
selection rules, which are crucial for the single-layer lattice, may become
lifted in bilayer lattices. The results for effective and tight-binding models
are found to be in qualitative agreement for some of the stackings and the
reasons for the discrepancies for others are identified. Our findings propose
optical conductivity as an effective tool to distinguish between different
stackings in bilayer dice lattices.
