I searched this topic on the web and through this site and came across someone who had a similar issue. (Topic: "Urgent Decoder 4x16"). I tried to implement their answer for my circuit, but to no avail. Instead of using 74LS138 I am using two 74155A IC's. I also have an attached picture as to how I currently have the decoders wired. If anyone could give me some input, I'd really appreciate it

 

Too bad no one enlightened us yet.

 

Using a '155 is a bit harder than using a '138. The enable lines have to be used to let the selection lines select between the "a" and "b" sections. That calls for some external gates to enable the sides using the C select line. You do something similar for the D line to control the 16 line decoding.