It is commonly known that loading your carabiner in three directions is bad, as it reduces it's strength. This is apparent when talking about belay stations, as e.g. described in this blog post by the American Alpine Institute. The same danger is often associated with attaching a carabiner to both the seat and leg loops of a harness as opposed to just the belay loop. I haven't yet seen any evidence supporting or refuting this. By evidence I mean something like an accident report or actual tests done.
My personal hypothesis is that this is not problematic. I am just stating this as a reasoning for asking for evidence. I do not claim that this hypothesis is correct, as equally I haven't done or read any tests validating it. These two loops are close together in the first place and their position is not fixed, they are only tightly wrapped around body parts, which themselves aren't rigid either. Therefore I believe that under load they will be pulled close together and away from the body, such applying an effective single force to the biner.
Also, this is not about any other dis-/advantages except this particular safety issue. I know people who use it, which reason enough to ask this question.