Multiple vents going into the same flue are fine so long as they’re burning the same fuel. I can tell you that I see many chimney flues in multi-families taking on 2 water heaters and two furnaces. That’s fine. Any more than that, I’d be suspicious.
As for your dating code, Prestons has a huge window in that particular model #. As you stated, it has the same model # from 1992 to 2000. Prestons always confuses me.
According to my HVAC records, I have a different formula, but your numbers don’t make sense.
Goodman: First two digits of the serial number are the year. Second two are the month. Btu rating is model number…ie; GMP075-3 (75,000 bonnet input) Compressor is same :serial # 9709088872 (yr1997 wk 09) Tonnage is model # CK361C The first two numbers after the CK (letters) divided by 12 is tons. ie: 36 div by 12= 3 tons.
Are you sure the first two numbers in the Serial # are 72? According to this code it’s got to be a 9. That’ll make it a 1992.
But then the week would be 55? We only have 52!!!
Multiple appliances into a single flue can be correct, but if you have a doubt always recommend a professional inspections, combining appliances requires a good knowledge of flue venting practices to properly size the piping especially if you have a combinations of natural draft and powered draft units. Look at the venting for signs of corrosion, water leakage (flue gases cooling and running back down the piping) etc