how can I separate these 2 fingers? I managed to sculpt a hand but the ring and middle finger are united half way.. How can I make a distance between them? My .blend file is here:

Here are some pics: 
This is the side view of the problematic part in edit mode
They are not united. They are overlapping.
Well, easy ... not really, no. First you need to select the finger you want to edit. I did from the inside, which I think, is much easier.
Then you can rotate, move, scale the selected nodes/faces. Proportional editing might come in handy. Also, using the 3D-Cursor at the nuckle might help with rotations.
You could also select the middle finger fist, then hide those faces [h] to work on the ring finger - or vice versa. Unhide all faces (in edit mode) [ALT]+[h].
Hint: when you hide faces, switching to sculpt mode will keep those faces hidden.
Also, while you're in edit mode: create a vertex group of each finger. This makes further selections and rigging easier.

Viewed from the inside. Left, selected ring finger. Right, not selected middle finger. They are overlapping at the &quot;first visible&quot; bone.