UE4 reflection system does not support array in array (even in C++, regardless of fact that C++ allows to do so), so it’s impossible to make multi dimensional array. But there very simple workarounds which makes single dimension array to work like two dimensional array, by playing allocation numbers. If one dimension has static size, you can multiply that size in allocator by number of y postions, like this:
x+(x_size*y)
Ofcorse you need to watch out to x not being higher or same then x_size or else it will read or set other y row. You can make functions to make it a lot cleaner and simpler