You should always be able to use multiple devices at the same time.

Normally the catch is that you're charged per device, so you'll get dinged twice if you use it on two devices, etc. Given that you get it free this shouldn't be a problem, but I'd keep an eye out on your bill just to be sure that they don't try and charge you for one of the two - in which case just ask them to remove it.

As mentioned above, a travel router will avoid the issue as it'll show up as a single device no matter what's behind it - but I'm really not sure that it would be worth the effort just for this.