I currently have Comcast TV and internet at my house, split off a single line coming into my basement. I am scheduled for install of business internet in a few weeks. I want to be sure that my business activity doesn't negatively impact speeds and signals on my personal line, and vice versa. Should I insist on a separate line being run from the box? Or is that unnecessary?
Technically it will just like your neighbors hooked to the same cable going back to the ISP. It does affect the speed no matter where it is connected. The actual transfer rates from the ISP on the cable is actually many times higher than what they sell you....unless they are real idiots. This allows say multiple people to have 250mbps when the actual data rate is say 1gbit or if its docsis 3.1 10g. They just assume everyone will not be max it out at the same time.
There is some tricky stuff they do with business but I think they just give it more priority over residential when there is contention.
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.