Bigender lesbian refers to a bigender person who also identifies as a lesbian. Bigender lesbians can identify as any two genders, but usually they are fin-fem-aligned / woman-aligned.
Bigender lesbians fall under the non-binary lesbian umbrella, though they may identify exclusively as one or the other.
If a bigender is related to a woman, girl and or feminine while also being related to a man, boy and or masculine saying that they are both straight and a lesbian, you may correct them as a bigender lesbian. However, this can also relate to those who are related to a non-binary gender such as demigender and or demigirl while also being related to a masculine gender relating to the term "non-binary lesbian". Though most of the time, they are bigender straights.