So I have had multiple cable modems from isps before, and even had one business one residential like you are planning to do.
No matter how they wire it, it will still go back to their demarc on the property. In other words, it's going to be combined before it leaves your property.
However, that only means that the cable signals are combined. These are modems so once the isp has a connection and it's good, it is up to the isp to push down as much speed as they want, typically up to what you're paying for. I never saw any slowdowns or ill effects from any one of my modems messing with another's speed with any isp I've had such a setup. And at one point I had 3x modems and was maxing them out for 12hrs every week.
However, what I have noticed is that if you lose your residential service, you can almost certainly bet that you'll lose business service as well. This is why I actually have a second line come from a different isp as the chances of both isps being down simultaneously are slim.
Hope this gives you some food for thought!