DDS with AD9833 and Microcontroller
The AD9833 has a standard 3-wire serial interface that is compatible with 
the Serial Peripheral Interface (SPI) standard. SPI support multiple devices with independent slave configuration. There is an independent chip select (SS) line for each slave. MOSI, MISO, and SCK are parallel for all slaves.

This connection should work:


ATMega328 PB3/MOSI to SDATA AD9833 (both of them in
parallel)
ATMega328 PB5/SCK to SCLK AD9833 (both of them in
parallel)
Any ATMega328 ouput pin connected to FSYNC first AD9833 as chip
select
Any other ATMega328 ouput pin connected to FSYNC second AD9833 as chip
select