Yes, you can register several Moments with just one account. In this case, you always have to select the watch you're currently wearing in the app, then connect it.
Can I use a Moment and an Orbit?
You can, of course, use both - but not at the same time. As you can only connect one device at a time with your Steps app, you have to choose one and then connect it every time you switch from Moment to Orbit or vice versa.