How to use two of this DC-DC converter synchronously to get more current output?
My first reaction was that you should use two of the controllers in parallel and connect the CLKOUT pin of the one to the SYNC pin of the other. It believe it is clear enough:


  SYNC (Pin 34): External Synchronization Input Pin. This
  pin is internally terminated to GND with a 90k resistor.
  The internal buck clock is synchronized to the rising edge
  of the SYNC signal while the internal boost clock is 180°
  phase shifted.


and


  Frequency Synchronization
  The LT3791-1 switching frequency can be synchronized
  to an external clock using the SYNC pin. The falling edge of CLKOUT corresponds to the rising edge
  of SYNC thus allowing 2-phase paralleling converters. The
  rising edge of CLKOUT turns on switch M3 and the falling
  edge of CLKOUT turns on switch M2.


You basically use the CLKOUT clock as the external clock to which the other controller is synchronized.

By searching a bit online, I also came across this in the LT journal, issue October 2012.



That should clear any doubt!