How to build a 4 to 16 decoder using ONLY TWO 2 to 4 decoders?
A \$2\$-by-\$4\$ decoder has two input lines and four output lines, only one of which is logical \$1\$ at any time.  Which line is \$1\$ depends on the input bit pair which can be \$00, 01, 10, 11\$.  

So take two such \$2\$-by-\$4\$ decoders which give you four input lines. Let the
output lines be \$a_0, a_1, a_2, a_3\$ for one decoder and \$b_0, b_1, b_2, b_3\$
for the other.  Use the \$16\$ AND gates to compute the \$16\$ functions
\$ a_i \wedge b_j, 0 \leq i \leq 3, 0 \leq j \leq 3\$.  We now have a \$4\$-by-\$16\$
circuit with the property that only one output is a logical \$1\$ at any time: which one depends on the values of $i$ and $j$ which
in turn depend on the \$4\$ input bits.  In other words, we have a \$4\$-by-\$16\$ decoder
constructed from two \$2\$-by-\$4\$ decoders and \$16\$ AND gates.