And maybe the picture of the board would help, which I was able to upload.
It shows there is no functionality to support another iron.
Maybe something like, the transistors, resistors, capacitors, triac, pcb etc serves these functions and capabilities and therefore they aren't capable of supporting an additional soldering iron.
How easy do you think it is to attach another pair of wheels to a car which has no provision for such upgrade in it's design?
I guess you're not seeing it from my level. If you tell a toddler why they can't do something, they'll always respond with why? Things my be obvious to you and many here, but not everyone, but yeah I can accept there's no functionality, but I'll never know exactly why.
everybody with experience here should agree with wraper. in case of a car, if you want to add 5th wheel, you will not just add the wheel, you have to add the arm, absorber, bolting to body everything. in case of your station, you have to add the optocoupler, power elements (fet or/and triac) and everything identical supporting elements resistors, diodes everything to your pcb. knobs to control, and modify firmware if your station have one mcu. can you do that?
in case you cant (which all of us suspect you will as we all wont dare get into the hassle) we always have another way, you can parallel port connection, ie power wires of your soldering iron, but you cant parallel heat sensor's (sort of resistor divider) wire because of Ohm Law, your iron temperature will go either extra red hot or blue cold. and you also risk damaging your power elements because 1 power elements can only have burden 1 solder iron. if you put 2, it could breakrendering you have to buy a new one.
of course we cant stop you if you still want to try to see what will happen and find a reason that we usually have encountered. cheers happy learning.