Teeth and pliers tend to leave unsightly marks on the bricks.
Fingernails don't work too well.
What tool do you recommend to separate them?
Or you could use, guess what, a new brick separator:
In addition to the Brick Separator, there are some parts that work well for this
There's the great LEGO Brick Separator that can be bought in the LEGO Online Shop:
I love this thing. It's just great if you want to take care of your nails and teeth.
I (and my sons) use a plastic knife designed for children. We found ours in IKEA (larger bricks for houses)
I have also used this lego tool with moderate success
You can always try the Official LEGO Brick Separator. It would probably take a couple to work best on 1x2s though - one on the top, one on the bottom.