POLL: What did Apple CEO Tim Cook and Google CEO Sundar Pichai talk about over dinner?

Oh, to be a fly on that wall.

Rarely spotted together, CEO and CEO were seen dishing over Vietnamese food in Palo Alto this week. Apple and Google parent Alphabet are the two most valuable companies in the U.S., with a combined market capitalization of over $1.5 trillion.

Source: Steve Sims

Steve Sims, founder of luxury concierge service Bluefish, was hosting a corporate client at Tamarine in Palo Alto on Tuesday when he snapped the shot on a smoke break around 9 p.m.

Sims didn't have much to share on the conversation between two of tech's most powerful men. But one can be sure, as Cook would say, that it was the "mother of all" conversations.

The two companies have a long-standing rivalry — stoked not only by their competing mobile phone platforms, but the lawsuits that followed.

Still, the companies also have a great deal in common — Google's Eric Schmidt, then CEO, even sat on Apple's board until 2009. Google backed Apple on its encryption battle with regulators last year, and recently the companies have faced similar challenges, including immigration reform and a dump of Wikileaks files.

CNBC is reaching out for any potential comment on what was discussed — but you can vote below on what you think.