Obviously if the LLB grad also has a non-law degree (BA, BCom), he or she is exactly as qualified as the undergrad --> JD graduate. The only thing distinguishing them would be any personal qualities or experience that tend to occur in one graduate group, but not the other. For instance, JD graduates may be more likely to have worked in a different field before obtaining the professional degree. Clearly this has little to do with the qualification itself though, and is largely irrelevant in this sense.
For me, the more pertinent question is: does the BA/LLB job candidate - or, for that matter, the BA --> JD candidate - have an appreciable edge over a candidate with a straight law degree? Logic would dictate yes, because objectively they have "more qualifications", but I wonder if law firms actually see this as being greatly beneficial. (I assume that, by "employability", the OP is referring to employment arrangements in law firms.)
I guess in the case of BCom/LLB or BCom-->JD the answer to this question is more straightforward, as firms might look explicitly for someone with an accounting/finance background or w'evs.