Have you considered a Wi-Fi extender? I use one I bought at Aldi several years ago, and it works great. They don't have to be expensive, I only paid about $20.00.
Is it possible to have two modems in one home?
If you mean two straight cable modem or two combo modem / router gateway devices, Most systems no longer allow two modems to be used at one address. It's best to call in to see what the local policy is. Even if they do allow it, you will need to pay for two accounts (twice what you are paying for HSI now).
Powerline ethernet or MoCA adapters may be an option for you. Google for more info.