It's not a stretch at all. BD1 and BD2 serve complementary functions. By inhibiting one, you are still able to inhibit the combined function. 

“In the case of human BRD4, the first BrD appears dedicated to anchoring this molecule and its associated proteins to target gene promoter and enhancer sites in chromatin, through binding to diacetylated H4K5ac/K8ac (a mark for gene transcriptional activation), while the second BrD was associated with the recruitment of nonhistone proteins (i.e., transcription factors and the pTEFb complex) to target genes.”

https://www.sciencedirect.com/science/article/pii/S1074552114001793?via%3Dihub

Other point on "human CI mutant cybrid cells" is well noted.

As always thanks for the insight and speedy response.