You can find out which one is bisexual by asking them directly. The problem is that one of them will lie about it so they will both give you the same answer.

By asking one of them “If I asked the other one if you lie, what would they say?” you’re able to figure out which one lies and which one tells the truth and then all you have to do is ask one of them to figure out who is bisexual.

Let’s say left tells the truth and right lies. If you ask left, “would right say you lie?” She will answer “Yes”truthfully, knowing right will lie. Right would lie and say that left will lie.

If you started the other way around and asked right, “would left say you lie?” She will answer “No” untruthfully, knowing left will tell the truth. Left would tell the truth and say that right will lie, so she gives the opposite answer.

So basically, if you ask either of them, “would the other say you lie?” If they say yes they tell the truth, if they say no they lie. Then you just follow it up with “are you bisexual?” And now you know the answer.