I want to make dent in some models. However, when I do it, I can only do it to one side at a time. Here:

If you look closely, I only did it to one side.
However, I want it to make it completely symmetrical, instead of doing the other side again and trying to measure out the same distance for the dent as for the other side.
Is there any options for symmetry and/or so the 2 selected faces go away from or to each other when you grab it to move it?