Is it a problem to have a fleishik oven on top of a milchik oven or vice versa? Many houses have it set up that way.
It would depend on the model and type of oven:
If they share vents, you cannot use one for fleishig and one for milchig. This is because the steam from one oven goes into the second oven1.
If they do not share vents and the two compartments are attached with screws or wires (as it is in most cases), you cannot use them at the same time for milichig and fleishig.
To cook one after the other would be permitted, but some are strict to wait 24 hours.
If they are not attached but merely one on top of the other (or just the outsides are connected) – as it is in some models – there is no problem to designate one for milichig and one for fleishig.
When cooking at the same time, you should be careful not to open both doors at once so the steam from one oven does not go into the second oven2.