I need to mount a 2 gang dry lining backbox on a drywall. 
Now, the hole in the wall is too long, so the clips won't grip (or rather, only one clip will grip where the other one would be too far from the drywall edge).
For reasons beyond my control I can't move the box. Any suggestions?
Electrical – How to mount an electrical box in a drywall hole that’s too large
drywallelectrical
Related Solutions
You're going to have to do drywall repair if you move this switch.
So since you HAVE to do drywall repair regardless, why shy away from doing it the right way?
Cut away a big enough piece of drywall so that you have full access to the existing box, the space where the new box will be, and keep cutting right and left until you hit the bordering studs on either side.
Do your electrical work.
Cut 1 piece of drywall to fit the huge gaping hole you made. Cut out space for your new box.
Cut pieces of 1x2 to fit vertically on the insides of the left and right studs, and to fit across the tops and bottoms of the opening. (6 pieces total)
Install the vertical pieces parallel to the studs with 1" side facing you, so you have a screw edge. Install the horizontal pieces so that the 2" side is facing you and half of it is behind the existing drywall. Screw it in place in the existing drywall.
Take your cut piece of drywall and mount it on the 6 1x2"s you just installed.
Tape and mud - 3 coats - fill coat, feather, final.
Have you thought about trying to achieve the same result without running wires? Assuming you're using a cable (or satellite) box of some kind (and your TV doesn't have a built-in cable card), you could use something like this:
That would give you a standard RCA connection from your satellite/cable box to your TV. There are other options for other connection types; e.g. this one for HDMI (although it gets spendy).
Failing that, how about just using a quarter-round cable cover around the edge of the room?
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Best Answer
I assume you used this box to avoid tearing up a bunch of drywall. So let’s try for a solution that avoids that.
Mount a piece of wood inside the wall along the edge of the opening and mount to this piece of wood.
The wood must be small enough to fit inside the existing hole. Once in you rotate it into place and then mount with drywall screws going through drywall and into the wood. If this area is exposed wall (not covered by a picture, a TV, or other furniture) you will want to patch the screw heads.
Mounting to the wood may require some more DIY finesse. With so little room to work you may find you need to modify the clip attachment and find some other means to attach on that side of the box. Like maybe snip off the clamp part so the hinged piece sits flat agains the wood and then drive a screw in horizontally.
HOWEVER, by the looks of things this is a box intended for main electrics? If there are high voltage (>50V) connections in this box it is a code violation to modify it. In particular, it may no longer be fire resistant after modification.