Electric baseboard heaters -wiring 1 thermostat to two heaters

I am using only one 240V #12 2 wire circuit running two heaters at a total of 3500 watts (one heater is 1500 and the second 2000 watts). I'm using 2 pole 20A breakers and 2c #12 wire.

My plan is to have both heaters controlled by one wall mounted double pole thermostat.

So far I ran my one 240V line from my panel ( a sub panel in the garage see note below)to the thermostat. I attached, the line in to the lead at the stat then the two lines to the heaters were pigged tailed together and pigtailed to the out source of the stat and attached to the stat. Before, I wired in each heater I tested the lines and they appeared to work: when positioned at off no hot wires and as I turned on the stat at low temp only one wire was hot and as I increased the stat temp another click at the stat and the second wire tested hot so I thought great. I then wired in each heater. Each heater had two wires which I attached and I grounded the bare wire. Turning on the stat resulted in no heat at all from either heater. What could be wrong? Where do I start checking?

Note this panel is a sub panel in the garage running three 15 amp circuits plus the heaters. The sub panel is running of the main using a 2 pole 30A breakers.

Thanks for your help

Everything you explain makes perfect sense and should work.

Be sure to have 240 volts at the Line of the stat, and once turned on all the way 240 volts at the Load of the stat.

A common mistake is not using the correct two wires as Line and Load. Two black are usually Line and Load is two reds on the stat.

An update
TK Russels point was bang on

"Be sure to have 240 volts at the Line of the stat, and once turned on all the way 240 volts at the Load of the stat."

The problem was my breaker was plugged into only one side of my load which meant I was getting 120 on each but not 24 combined.

The fix required that I had to move my breaker location and plug my dual pole braker into my pannel were one braker ran on the white line and one breaker on the black therefor running off both leads from the pannel and resulting in producing 120 plus 120 240 volts.

An update
TK Russels point was bang on

"Be sure to have 240 volts at the Line of the stat, and once turned on all the way 240 volts at the Load of the stat."

The problem was my breaker was plugged into only one side of my load which meant I was getting 120 on each but not 240 combined.

The fix required that I had to move my breaker location and plug my dual pole breaker into my pannel were one breaker ran on the white line and one breaker on the black therefor running off both leads from the pannel and resulting in producing 120 plus 120 240 volts.