If your goal is to study the number of live and dead cells (cell viability), you should use ethidium homodimer (EthD) along with Calcein-AM. Calcein-AM is basically an esterase substrate that is taken up by live cells and due to the esterase activity, it is cleaved to yield the fluorescent dye. It cannot stain dead cells as dead cells lack esterase activity or the dead cells might have compromised membrane which cannot help retain the calcein. This is the reason EthD (stains dead cells) is used in combination with Calcein (stains live cells). I have attached a few product descriptions for your reference.