Hi gherbi
I think you can do it even by using a single transmitter and a single reciever.What i mean is that you use a tunable transmitter.The same transmitter can be used for transmitting different wavelengths and use a tunable reciever at the reciever end which can be used to tune itself to a particular wavelength.
Although the best way is to use multiple transmitters and recievers but tunability will not limit the number of MIMO transrecievers in your design
Hope it helps
Regards