Going from old analog thermostats to programmable digital. They're all 240v but appear to currently be wired as single pole and new stats are double. Photos attached of line in, and two out for load- this one runs two baseboards. New stats are Honeywell double pole. I think I just need to separate the nuts connecting the white wires from the line in and run that to the new stat, leave the two white load lines and connect those to the load out of the double pole but wanted to make sure. Couple pics attached.
Your smart 'stat needs power. That means you absolutely need a 2-pole connection to the 'stat.
Generally they go for double-pole for safety. With single-pole, the guts of your heater are energized at 120V all the time, with the other pole of 120V connected when heat is called for.
That white wire is not neutral, it's a hot wire - it's white because they don't make /2 cable in black and red. That's why it's marked with black tape.
I would say make a "safety upgrade" to 2-pole, regardless of the situation with your 'stat.
With a smart thermostat, You'll have to anyway. Smart stats don't work on the muscular flex of a bimetal strip. They power themselves with actual power. The only way it can get power is by being connected to both poles of 240V. So you are forced to a 2-pole connection regardless. Congratulations, you are the owner of a spare wire nut.
I think I just need to separate the nuts connecting the white wires from the line in and run that to the new stat, leave the two white load lines and connect those to the load out of the double pole but wanted to make sure.
Short answer is: Yes, you can do that or you can just use one set of wires like they did. (The National Electrical Code does require a disconnect for the heater. The thermostat can be used for this as long as it disconnects both ungrounded conductors and has a marked "off" position.)
Long answer is: Whether you use a single pole or double pole thermostat it will work the same. It is a single phase circuit and it will work either way.
The difference is, with a double pole thermostat you are breaking the circuit in two places at the same time instead of one. This reduces the amount of pitting on the device contacts. With a double-break device the contacts will last longer at any given voltage. At 240 volts contact pitting is much higher than at 120 volts.
If you had a triac based thermostat contact pitting becomes a non-issue and you can go back to breaking the circuit in one place.