Looking at the oven on the Neff site it seems the height is split up into 96mm for the display panel and the rest is split 1/3 for top oven and 2/3 for bottom oven. Also, top oven capacity is 34l, bottom oven capacity is 71l. Taking into account the gap between ovens this makes it approx. 1/3 and 2/3 split.
So height 888 - display panel 96 = 792mm
792/3 = 262mm
262 x 2 = 524mm
So if it is a 1 third and 2 thirds split the top oven door is approx 262mm tall and the bottom oven door is approx 524mm tall.
If it is a 3/8 and 5/8 split they will be top - 297mm and bottom - 495mm.
It might be worthwhile phoning or e-mailing a retailer (eg John Lewis) and ask them to take a tape measure and measure the door itself for you to get an accurate measurement, most places would be willing to measure for you, especially if you indicate you might want to buy it from them (even if you have no intention of doing so) as they will help you out in the hope of getting the sale.
If, as a pp says, it is only about 450mm that will make the top oven door 324mm. Looking at the actual photos on the Neff site that doesn't look right for the ratio, but I might be wrong. Of course, it all depends on the angle the photos have been taken.
But, it does look like a lovely oven and if I had the money and space I wouldn't mind one of those. (Ever so slightly jealous emoji, looks at my old cooker with disdain.) (Hope you enjoy it!)