two vents in one flue

I’m taking the chimney inspection course and it states" the general rule is one flue for each heating system. But it goes onto to say " you may find a gas fired furnace and a gas fired hot water heater connected to the same flue. However, it does not go on to state whether this is an acceptable practice.
This is a common practice in my area and I see this situation a lot when I’m working on houses (as a contactor not as an inspector) . I have it on my house and there are no problems with it. The question is: When I start doing inspections, should I recommend this be changed ,or a least reviewed by an HVAC technician, or consider it an acceptable practice and not mention it on my inspection report.

It’s called common venting, and it is acceptable, but there are things you’ll need to look for. It’s not uncommon to find improper installations.

The water heater flue should connect to the common flue above the furnace flue connection point. If you see that, it’s OK.