Does Breakneck Deflect stack if I have 2?

Yes, the Deflect granted by multiple copies of Breakneck Mech (or any source that does not explicitly forbid it) will stack.

According to Rule 735.2:

"If a Unit has Deflect, or has been granted Deflect, and is granted Deflect by an additional source, the Deflect Value of all granted Deflect keywords is summed."

Since Breakneck Mech grants "Deflect" (which defaults to a value of 1 per Rule 735.1.b.3), having two copies of Breakneck Mech on the board will result in your Mechs having Deflect 2. This means an opponent must pay 2 additional Power of any domain to choose one of your Mechs with a spell or ability.

Note on Exceptions: Some cards, like Spirit Refuge, specifically state "they have Deflect if they didn't already," which prevents stacking from that specific source. Because Breakneck Mech lacks this restrictive wording, it follows the standard stacking rules for keywords like Shield, Assault, and Deflect.