Factory dual fans on the newer Suburbans/Silverados have a low/high speed that is controlled using a three relay system. Fan 1, Fan 2, and Fan Control. The PCM controls that with two ground signals, one for Fan lo and one for Fan hi. AC pressures or coolant temps will trigger the low speed which turns on both fans running in series. This is a pretty low current pull and doesn't really need a 'soft start'. When coolant temps get into the 'hi' territory(over 215 degrees) then the Fan hi output triggers and switches both fans to parallel operation. This doubles the power draw, but since they are already on it also doesn't really surge the amperage that much.
Hi speed should never trigger without low speed being on first, and low speed is a small enough power draw that it can be run almost continuously without overloading the alternator and is usually sufficient on it's own to keep temps under control and the AC working fine. Hi speed usually only runs when it's crazy hot outside or you've had the engine under a pretty heavy load for an extended amount of time.
This is a great control strategy that works very well. Here in Texas, high speed kicks on more often, but you hear the air more than feel the fans kicking into high. Running both fans in series absolutely keeps the inrush current down, eliminating that "pull down" effect when fans kick on, and going from lo to hi also doesn't have the surge because the fans are already turning.
Another way to do something similar is to have a low speed resistor from something like a 90's FWD GM car, especially if you are using a late 90's fan setup (the connector is compatible, it changed somewhere around 2005-ish?). Those FWD setups had one main fain, and low speed was accomplished with a resistor inline and high speed is straight 12v. Some had a smaller secondary fan, maybe a 4 cyl vs V6 difference? It was controlled separately, but I assume with high speed. Cadillac's had a double resistor, so that fan low is super quiet and you can hardly tell when it turns on at all. Buick and Chevy had a single resistor that is still very nice, but makes a little more noise on low. Switching to high speed bypasses the resistor, and it all controlled within the harness. If you can get into the early 90's cars, you should be able to get a complete harness with double relays as a standalone that is great for adding to a non-E-fan vehicle, newer ones used relays in the fuse block.
I'm still running mechanical on my Suburban, but when I swapped my Monte, I wired in one of the dual fans with a low speed resistor that I tied to ignition power. When the computer calls for fan 1, I think I had it send full power to the first fan, and run low speed to fan 2, then high speed it ran full speed to fan 1 and 2. It never has heat issues, and rarely kicks on the first fan stage, although it does not have AC. If you live in a mild climate, having AC trigger low speed is probably fine, if you live somewhere hotter, high speed will help get your AC temps lower on a hot day for sure.
Here's the fan type I was talking about, the resistors are the white blocks. 3 wire incoming, motor is still a 2 terminal standard motor.
