You have to be careful with this kind of 'I'm on' signal.
A lot of regulations for un-licensed radio frequencies (such as the ISM band, etc) stipulate a maximum duty cycle on the transmission, meaning that you are only allowed to actively transmit for a certain % of the time. This prevents blocking of a particular frequency and allows better TDM of signals.
For example, for the ISM (Industrial, Scientific, Medical) range:
Dependent on the sub-band the transmission power is limited to 10 dBm … 27 dBm. The permitted time allocation (duty cycle) also varies with the sub-band. So interferences with other 868 MHz equipment are reduced and thus a better transmission quality can be achieved.
- ISM 433/868
So whatever solution you go for you will have to create some kind of periodic pulsed beacon that sends a short burst signal at pre-defined periods rather than just a constant 'on' signal.
This should also mean that you can save power in between the pulses as you can turn the transmitter off (a lot of TX chips have a 'sleep' or 'shutdown' mode) when it's not actively being used.
As has been mentioned in some of the comments, there is most likely a set of reinforcement bars in the concrete forming a 'Faraday cage'. This blocks a large number of signals from penetrating as they hit the bars and then get grounded by them. So, you need to pick a frequency that has a wavelength that is smaller than the space between the bars. Also, concrete can absorb the signal at certain frequencies.
From what I understand this is mostly due to the water content in the concrete. Water contains hydrogen. Hydrogen resonates at 2.4GHz. Many transmissions (WiFi for example) occur at 2.4GHz, so the hydrogen in the water in the concrete absorbs the transmission.*
So, the ISM 868MHz band has a wavelength of about 0.35m - this is probably going to be way too big to fit between the bars (I'm not sure what the regulations state about bar spacing). ISM 915 takes it down to 0.33m - still too big. The 2.4GHz is 0.125m - much more realistic but may not get through the concrete. So you'd be looking somewhere in the mid-to-high 1GHz range. Personally I'm not aware of a license free frequency range in that area. You'd need to check with the RF licensing people in your area (FCC, OfCom, etc).
*(This is purely my own conjecture - please correct me as I'd like to know the truth myself)
There are different types of MIMO. Those are Precoding, Spatial multiplexing, and Diversity Coding.
Precoding
The idea behind MIMO is that at the frequencies being used, the wavelength is small enough that even 30 cm apart is enough to receive the signal at different phases. As Brain said, the wavelength is about 12.5cm for 2.4 GHz. This means that regardless of how far you are from the two antennas, the delay (or phase delay) between the two antennas will always be fixed for any given angle.
You are able to take advantage of this phase difference to create beam steering. The math and actual implementation of this is complex, but the general idea is actually relatively simple. If the two signals are in phase, then you know that the source of that signal is the same distance from each antenna which means that your source has to be somewhere along the line of symmetry.
As the source begins to move around, the signal will get to one or the other antennas first and the angle from the receiver can be determined based off of the amount of delay between the two. This then allows you to setup "sectors" or beams based of off how much delay is applied to the incoming signal.
Now technically the drawing I showed is only MISO (Multi in single out), but the logic holds true when you add another antenna to create a full MIMO. Also, on the transmitting side, you can do the same thing I talked about with receiving, but instead a delay is applied to one or the other antenna to create a beam in specific direction out of the transmitter.
The accuracy of the angle in and out of each pair of antennas is determined by both the spacing of the antenna and the accuracy of electronics to produce and detect a specific phase shift.
Also things get more complex as you start to account for the fact that at some locations the signal might appear to get to the antennas at the same time but are actually 1 full cycle apart. Also there has to be a control system setup to know what direction you should be directing you beam at, especially when you have a moving device.
But to get to your question directly, it doesn't matter if your source has 2 antennas or not, it is treated the same on the receiving end. What maters is the angle that the source is from the destination. You essentially end up with a source directing its beam in the general direction of the receiver and then the receiver is steering its beam in the general direction of the transmitter.
The big advantage of using MIMO is that you are not creating a lot of extra noise for neighboring devices and so you are able to get more devices in to a small area. Also, since the signal is more directional there is less to bounce off of which results in less issues with multipath.
Best Answer
When an isotropic antenna transmits power it does so in all directions. At some distance R all that power is passing thru the surface of an imaginary sphere of radius R. At distance 2R, the same amount of power is passing thru the surface of a sphere of twice the radius etc etc..
An isotropic receive antenna placed at some distance to the transmit antenna can only collect a small fraction of that power. It has, what is called, an effective area (or antenna aperture) and this is measured in sq metres.
This means that two isotropic antennas (that don't overlap) can each collect their own power and the transmit antenna doesn't know - it believes (if it had the power to reason) that all the power transmitted is going to reach the far ends of the universe.
The same is basically true of any antenna - providing the effective apertures don't overlap they'll each collect the "theoretical" power transmitted by that sort of antenna.
All this falls down when you get to the near field because in the near field you can effect the transmit antenna by loading.
Think of it like a light-bulb - several people in the room can each see the light but if one person stands in front of another then somebody misses out.