Depends what you mean by lethal.
If you take enough of each in combination, depending on your own physiology and medical history you may depress your central nervous system so much that you stop breathing.
You may also get so drunk and sedated that you vomit and aspirate your own vomit if you're lying on your back or another position where that is possible.
But either scenario is a gamble. It's not a reliable way to CTB.