I’m a little suspicious about the threat of “serious harm” to your modem, and I actually expect the gateway address to be the same.
But whether or not it should work, or whether or not you’re allowed to have two IP addresses … that’s up to your ISP.
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I’ll start with the modem. Normally those that supply two different connection types, such as USB and ethernet, expect that you’ll use one or the other, but not both at the same time. Some may, but I wouldn’t count on it. In any case, it’d have to be a very poorly designed modem if trying to use both caused it damage. At worst I’d expect one or the other to simply not work.
Now, whether or not you can have two IP address is totally up to your ISP. They may arbitrarily limit it, typically to one, and usually charge more if you need more IPs. It’s also possible that attempting to use two on the same connection can cause confusion, especially for incoming connections. Again, that’s totally at the whim and configuration of your ISP. In general, though, unless you’re paying for more than one IP, you can probably only use one.
Now, the gateway IP address is fairly simple. That’s just the next computer or router upstream from yours that is the “gateway” to the internet. When you communicate to another computer on the internet, the packets you send are physically sent to that gateway as their first step out onto the internet. It’s surprising to me that you got two different gateway addresses, but certainly not impossible.
My real recommendation in the situation you describe is to bypass all these questions and work with a more common solution that’s designed specifically for what you’re attempting to do: a NAT router. Routers are not that expensive, and basically sidestep all of your questions. They’re designed to take that one IP address and allow you to share it with two, or more, computers.
It’s a much simpler solution.