Help resolving uneven heat in duplex (NJ)

[Brittany Edwards chu]

Hello! My duplex in north jersey is 100yrs old, and it has steam heat/radiators. The gas line isn’t split out, and unfortunately only the first floor tenant has access to the thermostat. The top floors don’t heat unless the thermostat is way up. I’ve been meeting with hvacs to determine best solution. One option is averaging temperature smart thermostat, and wondering if anyone has experience with this? I’ve read it doesn’t necessarily work, but figured I would try before splitting out the line. The alternative is to give each apartment its own line, with new smaller furnaces, getting rid of the existing furnace that’s too many btus for just one line. House also occupied and don’t want a major disruption. Any suggestions on how to approach this would Be greatly appreciated!