Just some random thoughts..
We can only pay full attention to one thing at a time. So the gameplay would probably have to rely on the switching the player's focus from one view to another at strategic times.
Also the controls might conflict between the two view types if trying to use them simultaneously so would have to consider that.
I can imagine the player navigating in the top down view as 'normal' (thinking of an exploration game here), then in certain areas it would require the player to navigate through that area via the side view for a bit (staying on the same 'tile' in the top view), until they 'get through' that area and move on again with the top view. However this treats the views as almost separate games..
It could be a puzzler mechanic, in which case there might be more scope for using the views 'simultaneously'.
Doing a quick prototype would probably get the ideas flowing