I actually don't get what you are asking by "double acting" ....
if it meant a "threesom"
Nop... they were normal girls.. not sluts .. so wasnt into that sort of a thing and didn't ask either
if it meant "acted as i mistook one for one"
Nop., they both knew I was screwing the other sister

and I was very clear of my intentions with both of them. I made sure they both know that this just a short time fuck fling and not a long time marraige type affair
it it meant "experimenting in different sex positions of likes and dislikes between them? "
yes.. and I think I was pretty vividly descriptive about it as well.
so.. as said.. again...
Physically both of them were the absolute same sans a little bit of manufacture glitches like a birth mark etc. It was simply as sleeping with the same woman but with different attitudes.

... from the smell of their skin to the taste of their pussy was identical. Including how it feels "outside" and "inside" as well.
Mentally.. just two very different women with needs and wants of a variety. HECK man.. one hates chocolate ice cream while the other loved it.
Hope that answered..... but do ask if it did not