It seems that most of the better DAPs have dual DACs, what’s the advantage of this, is one used per channel or is it for something else?
One per channel. Most DAC integrated circuits, e.g. Burr-Brown, ESS etc., are dual channel but can be configured for single channel.
I think it is marketing … charge more for including an extra $5 IC. They may argue better channel separation or SNR but it doesn’t follow that two is better than one.
Yes it does, and it is measurable! If you keep the channels completely separate i.e. dual mono, there is reduced cross talk and other artefacts that will impact on the headline measurements DAC manufacturers use to market their products. Wether we would appreciate and enjoy the differences can be called in to question but the differences can be measured if properly done.
I have had DAPs for years but the places they are used, trains, planes, city streets means that, for me, the sonic advantages of the higher end kit were lost. Even with noise blocking in ear phones such as Etymotic.
And now I have gone to true wireless buds with their limited CODECs the phone is good enough and a shed more convenient. So my twin DAC, twin amp DAP mostly sits in the drawer.
(Due to a slip between hand and pavement it’s not worth much second hand so i use it as a backup and occasional roon end point)
Obviously just my take on it.
I’m looking at it to replace my two headphone zones at home and to use on the move so for me its adding flexibility and cutting down on kit. I’ve done Bluetooth from my phone but I how fed up of dropouts which happens everyday in busy London. I much prefer my iems and DAC.
I’d look at the balance between home and out and about and then buy based on that. The more out and about go for compact and battery Vs home use where quality counts.
As roon is also promising a better Android experience I’d go for as vanilla Android as you can get for home use.
I know which dap I am after iBasso DX160 but was curious about dual DACs.
Please point me to the measurements. 
As I said, it doesn’t follow that dual chip is always better than one; if the implementation is poor it won’t make one iota of a difference. Or to put it another way, a well-engineered single chip solution can out perform one with two.