Why would a DAC have both Vcc and V+ power supply?
AVcc supplies the DACs, V+ supplies the output drivers.
AVcc is supposed to be very stable, and power draw there should be minimal, while a bit of fluctuation on V+ is inconsequential because the driver stages correct for it -- but this supply needs to be able to supply more current.
If you tie them together, loading one output will slightly change the DAC value on the others, so you get a bit of coupling between outputs through there.