Yes! You can use multiple IVY devices all at the same time, the most important thing is that the devices are synced regularly and that your data is transferred to the app. The app will intelligently merge their data so you needn’t worry about your data doubling.
You can add a new device to the app by going into the app’s Settings > Add a device, and once it is paired, it will be assigned under different names so you could tell your multiple IVY devices apart, as well as by the icons associated to each device. You can change the name to whatever you like best by selecting the device, and simply tapping on its current name.