Death from Xanax overdose is nearly impossible unless you've stockpiled a metric ton of it. I won't get into exact dosing, but you'd literally have to take several thousand times the generally prescribed doses to ctb.
Lexapro overdose is unpleasant, but not usually fatal either. It can, sometimes at very high doses, lead to serotonin syndrome which has the potential to be fatal but is not always by any stretch of the imagination.
I'm not a pharmacist, but the odds of successfully ctb in this manner by mixing the two isn't great. The most likely outcome is that your body will simply cause you to vomit them back up, and that you'll feel miserable for awhile. There's also the potential for permenant organ damage if your attempt to ctb this way fails. Also, if you fail to ctb this is an almost guaranteed way to get yourself voluntold to an inpatient facility and make it harder for you to get effective meds in the future.
I'd definitely look for another route if you're set on ctb.