It's possible to use Circuit on multiple devices. To set up a second device, you need to log in with the Apple ID or Google account you used to initially subscribe to Circuit.
Keep in mind that you can't share an account with another driver, it's not possible to use different routes on multiple devices at the same time.
Please note that subscriptions are not cross-platform, so if you are planning to switch to a different platform, please cancel the subscription first and then resubscribe on a new device.
If you'd like to change your payment method or cancel the subscription later, you'll need to do that on the platform you've originally subscribed on (Android or iOS).