...finding them together...
I like the way you think though you have a different take on the relationship of the two submissives.
You imply them enjoying each other where as I envision the submissive female enjoying getting the male in trouble with the dominant for any real or contrived transgression. (i.e. he's not allowed an erection and the submissive female knows that just a certain look from her will set him off).
The dominant is aware of her submissive's entrapment of the male and can decide whether to punish the male (to the delight of her female submissive), to punish the female (to the consternation of the male who fears "sister's" retaliation later), and/or both (together or separate depending on the dominant's mood.)
The submissive female may subconsciously desire interacting with a male, but she is not willing to admit it or initiate any sexual interaction.
She would have to be "forced" by her dominant before she would participate.