I have never done it with mine, but as far as I know, there is no reason it can not be safely.
The problems I see that may occur are:
- Gas Piston comes out of sleeve after firing, not cycling the second shot.
- Hot gas may be directed more towards your face.
Just be careful, and unless anybody has reason to believe it is dangerous, try it out.
*Take my advice for what it is worth, this is the interwebs after all!