Are there any way to set all Vout pins of the LTC2668 and AD5360 to output synchronous and in-phase? If there is a way to achieve it, how should it be implemented?
for the AD5360, the LDAC pin is able to update all the outputs with the DAC data register contents synchronously. Same with the LTC2668, basically all DACs with LDAC should be able to update their outputs synchronously. are you looking for a sine wave output or just a fixed value output transition?
The implementation is to write to all dac registers of the channels, then toggle LDAC to allow synchronous output updating. The time delay for your SPI communication would determine the interval for each output change and ultimately your output update rate.
If you're looking for a sinusoidal output, you could check out this newly released product LTC2688 (Rev. 0) (analog.com)