This sounds like Glory by Greg Egan, which first appeared in The New Space Opera (2007) edited by Gardner Dozois, and reprinted in Egan's Dark Integers and Other Stories. It is freely available online as a PDF, which is linked on Greg's site.
From Classics of Science Fiction
“Glory” is about Joan and Anne, galactic citizens of the Amalgam, visiting a world that has not yet developed interstellar travel. Their adventure begins with two ingots of metallic hydrogen, one made of matter and the other anti-matter. These ingots are sculpted with neutrons and antineutrons until they are compressed into a needle one micron wide.
I strongly recommend reading the story online just to experience the dazzling science fictional thinking of Greg Egan.
However, once Egan gets Joan and Anne to the planet of Tira and Ghahar, two rival nations of beings call Noudah, the story slows down and becomes almost mundane in its plot. Joan and Anne are evidently what humans become in the far future, and they can download their essence (mind, soul?) into any machine or being. They appear to the Tiran and Ghahari in Noudah bodies. Joan and Anne each arrange to be intercepted by the two warring nations. Their stated and honest goal is to study the Niah, a race of sentient beings that had existed prior to the Noudah on this planet, and who were premiere mathematicians of the galaxy. The Niah existed for three million years but had disappeared over a million years earlier, leaving only tablets with their mathematical insights carved into them. Joan and Anne somehow know that the Noudah are building dams on Niah sites and want to excavate them before they are lost.
This story starts out as ultra-hard technological science fiction, at which Egan is a master. However the body of the story is more focused on social and political aspects. I think the combination works well, but it might be a bit jarring to some readers, if they aren't familiar with Egan's work. Although he loves writing hard sci-fi, and does it well, the science and technology are always subservient to the socio-political factors.