I have a question about subtractors. Is it possible to design a more efficient 2-bit parallel subtractor than connect 2 bit-slices of 1bit subtractor?

 

I don´t really understand your question. Do you mean chaining two one-bit subtractors to make a two-bit subtractor?
Also you need to define what "efficient" means in this case. Power consumption? Speed? Complexity? Number of transistors? Easier to design?