Yeah, if you want to get the most out of Lancs, get both guides.
In most chapters of the BMC guide, the bouldering gets one sentence, saying something like: "The Lancashire Bouldering guidebook describes 20 problems at this venue." There are a couple of traditional bouldering crags included (eg. Brownstones is in, but not the sitstarts). And some of the Silverdale limestone is included, but will be covered better by the forthcoming Lakes bouldering guide.
The BMC guide doesn't cover many of the bouldering only crags and the bouldering guide doesn't cover the routes only crags.