Welcome to the Forum.
I can't help much with who built your gun but as for the set triggers I'll try to answer your questions.
Double set triggers like yours consist of the forward trigger which will fire the gun just like a normal trigger.
If the triggers are "set", it also releases the lock but it does this by releasing the rear trigger.
The rear trigger isn't a trigger at all. It is the lever for cocking a heavy spring that is built into the triggers.
To "set" this heavy spring, pull back with quite a lot of force. When the rear trigger is pulled far enough, you should hear a faint "click" as the front trigger latches the rear trigger into the "set" or cocked position.
Once done, the slightest pressure on the front trigger will release the cocked rear trigger.
When this happens, the rear trigger hits the sear in the lock and causes the hammer to fall.
The amount of pull on the front trigger is only a few ounces and it cannot be adjusted for pull weight.
It can be adjusted for travel, that is, the amount it needs to move to release the rear "set" trigger. This is what the screw between the triggers is for.
By screwing it in, it reduces the depth of the "latch" the front trigger has on the rear trigger.
Screwed in too far and the front trigger won't even be able to latch the rear trigger in the set position. In other words, you won't be able to get the rear trigger to remain cocked.
Screwed all the way out will allow the front trigger to fully engage the rear trigger when it is pulled.
As I said, this doesn't change the amount of force needed to move the front trigger when the rear trigger is set but it does increase the amount of movement needed on the front trigger to release the rear trigger.
I don't recommend having a finger on the rear trigger when you pull the front trigger.
If you do have a finger on both triggers, when the rear trigger is released it will move forward very fast and most fingers don't like to be whacked like that.
Hope this helps you.
OH! I almost forgot.
You can cock and "fire" the rear trigger as often as you desire without hurting the gun AS LONG AS THE HAMMER IS DOWN.
If the hammer is at half cock, setting the rear trigger and "firing" it can break your lock.
Never set and "fire" set triggers when the lock is at half cock.
Also, in order for a lock to work with set triggers it must have a special little toggle built into its tumbler. This part is called a "fly".
It's job is to keep the nose of the sear from entering the half cock notch as the hammer falls from the fully cocked position.
Because of this fly, the hammer CANNOT be lowered from the Full Cock position to the Half Cock Position without lowering the hammer almost to the fired position and then raising the hammer up to the half cock position.
Raising the hammer from below the half cock position is the only way to have the lock engage the half cock notch.