How to synchronize two USRPs B200

I am trying to initialize a communication between separate USRPs B200 for my thesis. In order to do it, i need to achieve some important goals regarding synchronization and simultaneous Tx/Rx, specifically:

1) Each board has to transmit and receive simultaneously for a really short period of time (hopefully hundreds of microseconds);

2) Every board has to be synchronized with the others and, when I look at the receiver's side, I would like to capture the signal from every board.

By reading online, I found out that I need some external hardware in order to acheive synchronization in the order of microseconds, and then I have to use the command "EnableTimeTrigger" and "TriggerTime". To acheive synchronization with GPSDO, which other external hardware do I need? (Right now I only have a GPS active antenna). Thank you in advance for your answers.

[Walter Roberson]

It is fundamentally impossible to syncrhonize two clocks to closer than twice the time that light takes to travel between them.

It is fundamentally impossible to keep two clocks synchronized, without continual correction. For example if one of them is 100 cm above sea level, and the other is 100.01 cm above sea level, then the differences in gravitational potential will result in the clocks running at different rates.