Most likely your secondary GPU fan won't be spinning because it has the wrong vBIOS. Each 980M has to has a different and specific vBIOS on the 18 for the fan control to work. I believe there is a vBIOS package for the 18 with 980M's on the Dell Support page.
I'm not exactly sure why the 980M's aren't being recognised though. When you say that they also aren't recognised in the BIOS, what does it show? If it only shows the Intel GFX then the laptop is set to iGFX mode so the Nvidia cards are automatically disabled. This will happen when there are no dedicated cards in the laptop or when you use the Function button in Windows to swap GFX mode.
Resetting the CMOS in the 17 and 18 is not as easy as the older laptops. You need to do an NVRAM reset. See link below. Just note that you do not need to remove the CMOS battery like it says in the post.
When you install the 980M's and go into Windows, check the Device Manager. If the Intel 4600 graphics is shown under Display Adapters try doing the Function - GFX switch (can't remember if it's F5 or F7 on the 18). The Alienware OSD driver has to be installed to get the GFX switch to work. If it isn't, install it and try again. This should prompt a reboot to swap to the dedicated cards. If not, you could have an issue with one or both of the 980M's.
The other option is to flash the unlocked system BIOS and set the video mode to PEG and force it to use the dedicated cards. Do this at your own risk.