I was thinking you had steam heat, seems more like a heat pump setup.

The problem you have is of the two thermostats the one with the higher setpoint senses its room temperature and controls its boiler to maintain that. That means the one with the lower setpoint is always satisfied and never brings its boiler on-line. I had the same problem with the office part of my shop and the working area of the shop. The only way I solved it was to move the two thermostats away from each other so that each primarily sensed the ambient temperature at each location.

Why now? Electronic thermostats are precise to a fraction of a degree. Even with the same indicated setpoint, you end up with actual setpoints are not identical and you end up having the higher setpoint control the entire show. Between the mechanical hysteresis and the inherent inaccuracy of the old electro-mechanical thermostats I'm not surprised that it used to work.

Solutions, they have all been presented to you. If using one location for a thermostat for the entire house is acceptable, then tie the two boilers together with a single thermostat. If you want to retain both thermostats then you are going to have to get more separation between them. For my shop it took about 20' to get the job done.

Thanks guys. Tomorrow I think I'm going to try to run both boilers off one thermostat.

This evening, with both thermostats set at the same temp, the east side of the house was warm, and the west side was noticeably cooler. I bumped the temp up one degree on the west side. An hour later, the west side is warm, and the east side is noticeably cooler.

I'm grasping your situation and putting them both on one might work ok but since its already split with two systems you might as well take advantage and do it the right way by moving one of the thernostats further away. That'll also provide the best comfort level. Some people spend a lot of money to get that sort of control. It wouldnt even be too hard you could replace one of them with a thermostat that uses a wireless temperature sensor like someone else mentioned.

ebay shows that older style thermostats are for sale:

https://www.ebay.com/itm/1261953615...MFFCA%2BWcg%3D%3D%7Ctkp%3ABk9SR6L_tOf-Yg

We should not lump all digital thermostats together - a portion are probably superior and have stable temperature measurements.

I would avoid Venstar brand,
even though they have extensive internal adjustments and “calibration” features in their menu of options

So to bring this up to date, I have bought another thermostat (my wife didn't want to see the holes in the wall where the old, last purchased, thermostat was). The old stat is not connected to anything, but it reads the room temp and covers the holes in the wall. All three are Honeywell non-programable heat/ac units. I also purchased 25' of new thermostat wire and some nail in wire holders. $28 total for the new thermostat, 25' of wire, and the wire holders.

The biggest challenge was the new thermostat and the wiring are all on my wife's 3/4s of the huge closet. It had to be wife friendly, if you get what I'm saying. I drilled new holes in the wall behind the new stat, and drilled a hole on the inside of the wall about 18" higher, at the closet hanger bar. Then I fished the wire through the two holes, and attached the wires to the inside wall just under the shelf, and ran the wire down through the closet in the corner, pulled back the carpet, drilled a new hole in the floor, through the basement ceiling tiles and the lower ceiling, and ran the wire to the boiler. I only had about 12" of the thermostat wire extra, which I pushed back into the closet wall. That long 1/2" drill bit sure made that hole in the basement ceiling easier then I expected. It was still nearly 4 hours of effort getting it done.

The new stat is in the back bedroom, on the northern facing (outer) wall of the closet. It is about midway between the three heat registers in the room (this one room used to be the kitchen and the master bedroom. There is a short register on the west wall (used to be in the kitchen and is on the center wall of the house) a register on the north wall (outside wall) and another register on the south wall (the original southern facing bedroom wall). When the house was converted to a single family unit, they completely took out the wall between the kitchen and the bedroom when they removed the kitchen stuff. Then they added the huge walk in closet (4' x 8'). That master bedroom is 14' x 20'. The thermostat is on the north facing wall of the closet, nearly at the center of the 20' length, and about 11' from the north outer wall. I'm hoping there is enough air circulation there. The room only has 2 walk through doors, one on the west wall that is normally closed, and one on the southern wall that is normally open, It connects to the hallway that goes to the front room, the front bedroom, and the bathroom, it is also along the side of the eastern facing closet wall.

Its all set up and it functions, now we will see how well it functions. Its off to a pretty good start, both back bedrooms are at the same temps right now (its 38 degrees outside), something that has not been since the new stats were first installed. I'll keep you posted.