schizophrenic duplex

Hi, I have an over under duplex with a gas HB smith boiler (steam) for the upstairs apartment, also a gas fired hot water heater for the upstairs. Both vented into the only chimney for the house. The downstairs apartment is heated by an older Smith oil boiler and is steam also. This oil boiler also has a coil inside for domestic hot water for the downstairs apartment. One major problem is that all three (gas boiler, oil boiler, and gas hot water tank) are vented into the single chimney. I've had a plumber tell me that I'm grandfathered because I bought it that way but that if I ever pull a unit I cannot re vent into the chimney (sorry if that's the wrong terminology) as Massachusetts only allows 2 units to vent into a single flue chimney. Also they are not fond of mixing fuel types which I currently have going on. Gas and oil vented into the same chimney. So that's the set up I have currently. I've been having major issues with the oil burner tripping for the last year now. I've paid several outfits to come and try and remedy it. New transformer, new thingy next to the transformer with the reset button, new mixing valve, and a cleaning and just recently a new filter. All this and it's still kicking off. The latest technician believes it's sediment and crud from the tank. what's the best way to get around the overloaded venting into the chimney and replace/fix this problem? Any ideas? Thanks