Am I going in the wrong direction, or can two or more mosfets run their Source pins to a common ground and not interfere with each other?
Source --> GND <-- Source
And if that is allowed, will
Gate --> 10kΩ resistor --> Ground <-- Source
also work in this setup? Or will the closest resistors to the power source drain energy from the mosfets farther away?
Breadboard layout designed with minimal use of jumpers & minimal footprint, so everything can be easily moved over to a circuit board & soldered. Mosfets are IRLZ44N. Solenoids are 12v. Pins 3&4 go to Arduino.
Nice! Once my header's get in, I'll give that a try.
One more mosfet question - if the backside of the drain (where the screw goes) is too close to (or even touching) the other mosfet's backside drain, can that cause an arc or other issues?
You did good. However you have no choice, the source of the each of the MOSFETs must go to ground; This is the best way to drive your load. Just be sure you use a solid ground and it is also connected to the ground of the Arduino. Connect the 12V power supply ground to the source then to the arduino. You probably do not need the FB diodes as the MOSFET you chose is avalanche rated. This rating allows you to eliminate the flyback diodes. They even tell you how much energy they will adsorb. You can check that rating against your solenoid current. You will need to capture this in a CAD program to make your PCBs. I use KiCad as it takes me from schematic capture to the Gerber files needed to make the PCB.
