Why can't the Restasis vials be used for two days instead of one?

They are designed like that ... there is no preservative in Restasis and if you use it the next day there may be a risk of microbial growth. As patients on cyclosporine may be immunocompromised it is very risky to reuse the contents of a vial, especially in the eyes.

https://www.drugs.com/pro/restasis.html

You could get more information from the manufacturer ALLERGAN.