If he puts a splitter in the co-axial cable in the house then runs a cable out to the garage with another cable modem ($60), I don't see why he would need another service.
My current ISP put a splitter on the co-ax cable between my cable box on the TV and cable modem. Would a second cable modem (I've confirmed that the cable company doesn't set up or otherwise modify an off the shelf cable modem) require another ISP subscription?
The way my ISP(comcast) works is they provide service based on cable modem MAC addresses. So if i setup a second modem, I would most definitely have to pay for a second service.
But really that is pointless with the fact that the OP would not be able to access resources in the garage via the house or vice versa.
Adding a second cable modem just makes things more complicated and is a waste when its really easy to just run an ethernet line to the garage.
While most cable ISP allows you to provide your own cable modem, they would have to authorize the cable modem to access the internet.... At least for my ISP, cablevision/Altice ... or something now is like that... they authorized your cable modem box by one of the MAC addresses... you can't just plugin any cable modem that you like and have service, you can actually get to their network... and stops at their provisioning (limbo) area... So if OP were to go this route, he/she might want to check with their ISP first, It's just not the same as good ole days of analog cable TV any more..... It has to be authorized, and most likely they wanted a additional monthly FEE ...
I would just run a separate 3/4 or 1" pvc between the buildings just for data / low voltage lines... that way I can future proof all I wanted....just make sure you use the right 90's and 45s made for electrical... and no LBs used bury under ground.. keep in mind also that if you do fiber, it can't handle sharp bends.... unless something changed ...past 10 years. Who knows what the next future of communication is after fiber, all we know, it could be compress hot air , and 1" PVC you can run an compressed air as well

...
IMO going fiber now under normal circumstance is a waste of $$$ on bleeding edge .... you just don't needed that much data speed.... but hey, you never know next live streaming garagepornhub might be in some guy's garage...
and WylieDisels brings out a point, that they are now 2 internet connection (lets say your isp allows it for a fee), it's harder to share files, unless you do some funky things like VPN/Cloud services etc things.
There are these MoCa over Ethernet things if you still wanted to run coax... but you better get some nice coaxs ....and not some run of the mill things from 99Cent stores..