Outer[Append, mylist, mysecondlist, 1]
{{{a, b, 1}, {a, b, 2}}, {{c, d, 1}, {c, d, 2}}}
Also
Outer[Join, mylist, List /@ mysecondlist, 1]
Flatten /@ Tuples[{{#}, mysecondlist}] & /@ mylist
Flatten /@ Thread[{#, mysecondlist}, List, {2}] & /@ mylist
Distribute[{mylist, mysecondlist}, List, List, Partition[{##}, 2] &, Append]
all give
{{{a, b, 1}, {a, b, 2}}, {{c, d, 1}, {c, d, 2}}}