Firstly, genuine 18-19 y.o.'s are about as common as unicorns. Any that you see advertised are probably at least a couple of years older than stated in their profiles.
If you are OK with early 20s then you should have no trouble finding a duo. That being said, if you want them to interact with each other, doing a lesbian show for you, that will be more difficult.
One London agency I have used in the past is [
00-44.net] - British owned and will actually listen to what you want and set up a duo with girls who are genuinely bisexual.
Good luck, I look forward to reading your review!