I found 2 way of making the 2x exporting work in affinity photo,
On a TRANSPARENT background layer, add a new layer create your element and scale it down.(you should be able to see dpi. Now 2 x exporting will be available ( without the warning triangle). It will give you ( if you save in 1x too.
a 1x image at your document size at 72 dpi a 2x image at double the document size at 72 dpi
The other way you need to know you final document size. You create you document at that size but you must be at 144 our 192 dpi. and on actual zoom pixel choose a device with (retina). Then paste an image, play with it, export will be available. it will give you 2 picture:
a 2x image at your document size at 144dpi a 1x image at the half of your doc size and dpi.
It is what Ii found this morning but I think this is really confusing. I'M not even sure if it is right or not. I think we need help with this feature in AP..
thx