Alright I looked all over Spark and AFF.net with no luck. I
think it was a fic that was originally started by one author, but then got taken over by another author (with the prior author's permission). Not sure about that though.
Anyway, somehow Sesshomaru and Kagome accidentally got stuck with this cube-type thing that allows you to jump to different dimensions. The problem is that they don't know how to work the cube so they can get back to their home dimension. So they just have to keep jumping. The second problem is that they have to wait for the cube to "reset" or something before they can make a jump and they never know when the reset is going to happen (could take 5 mins or 5 years).
Now that I write this out it sounds kinda like "Sliders" meets "Inuyasha".
One of the dimensions they ended up in was actually an "Aliens" dimension (yeah Ripley, facehugger, acid for blood Aliens). I know that sounds kinda crackfic-ish but it wasn't, it actually worked. They both plowed through the aliens like nothing since they really weren't that much different than the average demons they fought.
They start off as disgruntled allies but then after many dimensional jumps they start to catch feelings for each other.
Alas, the fic was unfinished (probably still is) but it I liked it primarily because it was kinda sci-fi-ish.