Running Avast on two computers with different operating systems

I use Avast on my PC, but my wife runs an old Mac for basic tasks. She would like to piggyback on my subscription. Is this possible and if so, how should I go about it?

First you don’t say what Avast Program you are using ?

The two Avast programs for, Avast for Windows and Avast for iOS are different to work on iOS.