Can you use a splitter to connect 2 fans to 1 controller slot? I have a controller that controls 3 fans and was wondering if a 4 pin splitter connecting 2 fans would be able to be controlled in one slot using one knob?
Depends, if it uses a simple resistor to control the output voltage - then yes or approximately. If it uses PWM then it's a function of the drive circuit.
Another contributing factor: Not all 12V fans have the same impedance. Need to look at nominal current drain. For example, if the fan controller will control a 240 mA fan, then it will work fine for two 120 mA fans in parallel.
I have 11 120mm fans in my case on a Sunbeam 4 channel fan controller that has an advertised limit of 20 Watts per channel. I have tried to balance the load of my fans so as to not exceed that limit but there is a noticeable but slight reduction in the fan speeds on the channels that are most loaded. I currently am using 7 fan splitters.
My setup at maximum 12V:
Channel 1 3x Scythe Slipstream 1900rpm fans estimated 19.8W load (0.53A per fan)
Channel 2 3x Scythe Slipstream 1900rpm fans estimated 19.8W load (0.53A per fan)
Channel 3 3x Scythe S-FLEX 1600rpm fans estimated 7.2W load (0.2A per fan)
Channel 4 2x OEM fans estimated at 5W load (one fan is 0.16A the other is 0.25A)
under normal operation the fans are run at about 7V
My point is that I have not had any performance issues with using fan splitters as I have stayed within the listed wattage of the controller channel (mostly). If you use a fan controller that has rpm sensing it probably won't display correctly but fan adjustment should still work. All of what I stated assumes that you are using a fan controller with 3pin fan connectors or an adapter to the 4pin molex. High wattage fan controllers usually have the wattage listed as part of the marketing. If the wattage is not listed prominently then the maximum is often 7W and I have seen as low as 5W per channel. I hope this information helps and as RetiredChief stated "Bottom Line - Try it"