Female Sherlock from a parallel Universe

[grypas.livejournal.com]

All I remember is that a female Sherlock appeared suddenly and that they switched realities by using the knife on the mantel.

ETA: Here's what I recall: both Sherlocks were fascinated with each other. They started examining each others body. They became aroused and proceeded to have sex. Male Sherlock asked if they were going to make a clone and female Sherlock said "shhh, you'll ruin the incest". When John arrived at the flat he found them comparing armpit odours and said "he drugged me again".

Found it with the help of Missilemuse and Grace
it's Double Trouble by iguana_dog