I think simulcasting means broadcasting on the same frequency from two different physical sites.
That is different than combining. Combining is a single controller broadcasting on more than one frequency (and possibly different physical sites on the "other" frequency).
For the CON 37 sector Boston Center seems to broadcast simultaneously on 134.7 from both Gardner, MA and Lebanon, NH. Not sure if they still do it...I know they do switch between the transmitters a lot, but there are times when you can hear them simulcasting.
The theory with simulcasting is that a given airplane will only hear one of the transmitters at a time. If they hear both, it can sound distorted due to the physical paths being different - the signals arrive at the plane's receiver with different signal strengths and phase.
-dave