Do you plan to sign in on the Alienware device with your Apple ID? If yes, can you try it now. I assume you have made sure your Alienware device's serial number etc. is unique and have followed the information in this or a similar guide -
https://www.tonymacx86.com/threads/an-idiots-guide-to-imessage.196827/
You can always remove the Alienware laptop from your iCloud account later if you don't want to use the Apple server functionality on the Alienware device.
What WiFi card/chip does the Alienware use if not Broadcom? Manufacturer and model number?
What kexts, patches etc. have you used to enable the non-Broadcom WiFi card?
Other than connecting between the two laptops not working, can you connect to any shared network devices (printers or network attached storage) from both laptops? I assume both connect to the internet without any issues.
If you connect both systems to Ethernet (assuming your MacBook has an Ethernet port or you have a USB or TB to Ethernet adapter) what difference does that make, if any?
I am trying to see if it is a specific WiFi issue or a general network issue