"Aren't you both also Americans" in Chinese?
你们（都）也不是美国人吗？你们也（都）不是美国人吗？
  
  Nǐmén (dōu) yě búshì měiguórén ma? Nǐmén yě (dōu) búshì měiguórén ma?
  
  Are (both/all of) you also not Americans?


In this case, you're inquiring your targets whether they are also not Americans, just like you or some other people.


  你们不也（都）是美国人吗？
  
  Nǐmén bu/bú/bù yě (dōu) shì měiguórén ma?
  
  Aren't you (both/all) also Americans?


This is a rhetorical question to make a point that you're under the impression that both/all your targets are also Americans, like you or some other people.  There are three possible ways to pronounce the 不 with bu being the most casual tone and bù being the most serious.

Gist: 也 is "also", 都 is "both/all".  不也都 signals that the question is rhetorical.  Depending on the exact sentence you wish to construct, you should omit 也 ("also") or 都 ("both/all").

As per the comments by user6065, it's also correct to use the following rhetorical questions if you're addressing exactly two people.


  你俩不也都是美国人吗？
    Nǐliǎ ...
  
  你们俩不也都是美国人吗？
    Nǐménliǎ ...
  
  你们两个不也都是美国人吗？
    Nǐmén liǎng'ge ...
  
  你们两人不也都是美国人吗？
    Nǐmén liǎngrén ...
  
  你们两个人不也都是美国人吗？
    Nǐmén liǎng'gè rén ...


These five rhetorical questions all share the meaning of "Aren't you two also both Americans?"