This is not allowed due to how the program activates your license.
For example if you purchased a 1-month subscription and you change it to the 3 month you will need to choose only one to use.
It is best to change your subscription a couple days before the subscription ends. We do not allow the same subscription to be used on multiple computers unless you have 2 separate subscriptions with a different email address and account associated to them.
If you attempt to purchase multiples it will cancel the order and refund you.