Yes, it’s possible to use coax cable to send a moca signal to another house.

Like running a coax line to your neighbor’s house so they can use your internet? Sure. A lot comes into play such as the signal strength by the time it makes it through the splices, splits, MOCA adapter and length of wire to the neighbor’s. What if you have issues from the ISP/wiring side? Well then good luck getting a service technician running that call when they see what’s going on.

There’s really too much of a hassle vs just the neighbor paying for their own connection lol.

You shouldn’t put signals into a network of lines that don’t belong to you, it would be better to run your own dedicated line if you want to go the MoCA route. It’s just asking for trouble

In addition to the other points about signal dropoff and general isp dislike, wouldn’t running an ungrounded copper cable between two houses potentially cause an electrical imbalance / risk of surge? That always gets brought up for why fiber should be used vs Ethernet between buildings but not sure if it applies here.

Do you have line-of-sight between the houses?

Use a wireless bridge instead.

I have three buildings connected together, one link is an Engenius 5GHz wireless bridge, and the other is a Ubiquiti 60GHz wireless bridge.