Multiple-Input Multiple-Output (MIMO) technology is a wireless technology that uses multiple transmitters and receivers to transfer more data at the same time. All wireless products with 802.11n support MIMO, which is part of the technology that allows 802.11n to reach much higher speeds than products without 802.11n.
For the most part, all MIMO devices use multiple antennas. There are some instances of single (or no) antenna setups, such asSISO/SIMO/MISO. However these are special case uses.
In older wireless setups, there was a single antenna, you can replace the original antenna with a high gain one. There were non-MIMO wireless receives with multiple antennas. You could replace one antenna with a high gain antenna, but it was recommended that you disable the other antenna, so that all signals went through the high gain antenna.
I dont know enough about the engineering behind MIMO, but it is obvious that the advantage it brings is from the multiple receivers and transmitters. You could replace one antenna with the high gain one, thereby increasing its signal/distance. Leaving the other antenna is stock would not hurt, but effectiveness would be diminished if there was a significant gap between the distance of the high gain and original antenna. It would make logical sense to replace all the antennas.