BREAKING NEWS – Looks like you can now glyph hack your way to getting up to 3 keys from a single hack!
You can get 1 key for the normal set of results from your hack, as well as up to 2 bonus keys.
A small scale test sample returned bonus keys 8 out of 10 times by doing MORE and COMPLEX during the control channel. Note that 2 of those times where a bonus key was received there was no key for the regular results, and vice versa. This roughly fits with the ~75% key drop rate that is normally found. None of the 10 hacks returned double keys, but several people have reported them in the bonus results as pictured above. It is unknown at this time what the rate of double bonus key drop is.
Ingress has confirmed this on twitter, as well as on G+.