Some of the answer depends on the GPU and CPU you have -- but if you want full performance out of the system, you must connect both adapters.

@davek456  Review of the Alienware Area-51m specify that you can use only one power adapter (I can't remember which one) for situations where you don't plan to use the NVIDIA GPU.  So I guess if you decided you wanted to take that system to a coffee shop to do some everyday work, you could get by with only one power adapter.  But it might have to be the 330W adapter; I'm not sure.  But if you want proper performance, you need both.  If you haven't already, you might want to head over to the Dell Support site, enter your Service Tag, click the Documentation tab, and peruse the User Guide to see if this is mentioned.

As for a docking station, you can use them, but as you've surmised, the docking station won't adequately power your system.  The USB PD spec maxes out at 100W.  Dell stretched that to 130W on some of their systems and docks, and even developed dual connector docking stations to push 210W to cover the needs of their Precision 7000 Series systems, but even that doesn't come close to what your system requires -- and to my knowledge, Dell has only implemented firmware-level support for using their dual connector docking stations in actual dual connector mode on those Precision 7000 Series systems.  So if you get a docking station, it will simply be to consolidate most/all peripherals EXCEPT your power adapter(s) onto a single cable connection to your system. Also be aware that to my knowledge, Dell has still not given their Inspiron or Alienware systems the necessary firmware-level support for using a dock Power button to control the system.  So even if you get a Dell dock that has one, like the WD19TB, you won't be able to use it to power on your system with the lid closed, for example.  No idea why they're withholding that feature, but they are.  And they're not alone in the industry, annoyingly.

Hi @davek456 

The Dell Support for Alienware Area-51m R2 is generic, using the system service tag does not customise the Documentation for the model.  

Setup and Specifications, Setup says: "Connect one or both power adapters that are supplied with your computer, it operates normally with one power adapter connected." So why the meaningless Pre-Boot "Alert....The system can only boot with a 510 power adapter or greater."? 

Setup and Specifications, External Ports says Docking port not supported. 

See Dell post Alienware Area 51 R2 Thunderbolt port? not supported. 

Hi @Wicky  if that was true, you would have provided the hyperlink in your post. 

Yes thunder does work....it's just a pain in to do it. Welcome to Dell....

I just posted this because I like my music...plugs

I hate that they bought Alienware.

And next time you need a hyperlink....look somewhere else

Dell support leaves something to be desired.....I hate that they bought Alienware.

I used mine for a second monitor, but yes the port works.