Of course. You can add multiple coats of Flex-Tec onto a piece of glass if you need to make a thicker coating.
You will need to wait until the first coat of Flex-Tec has become tacky before applying a second coat.
A good rule of thumb is if you touch the Flex-Tec with your finger and it leaves a fingerprint, but does not come off on your finger, then you can add the second coat. If the material sticks to your finger, you need to wait a bit longer.
The surface of Flex-Tec still needs to be slightly tacky for it to stick to the second layer, but not so loose that a second coat will disturb the chemical reaction of the first layer.