I am not a lawyer and I'm giving free advice on the internet, so keep that in mind. Also, it is exceedingly difficult to find specifics about this.
If you are using a paid account, ask your rep, whoever you purchased the first one from, whoever you got the code from or the EULA* you agreed to at download/retrieval of the OinkCode. They or it should have a better idea about the answer. I did find a hint that if you had multiple sensors you could ask for multiple codes, which implies you shouldn't/can't use one code for multiple devices.
If you are using the free one try reading the EULA*. If it's free, why not sign up for a second one? <-- again, the EULA will tell you if this is acceptable.
*I'm not saying that to be mean or anything 😞 - the EULA will tell you this answer and is the set of rules in effect for whatever it is you are doing. It might take a little looking, but the answer should be there.
One last note: Cisco isn't known for being a kind and gentle overlord. If I had to make a guess, just from knowing that I'd say you need to buy or register each separately.
