When Vin burned the 11th metal when first confronting the Lord Ruler in The Final Empire, she saw two men. I read the trilogy and I don't remember any reference of them later in The Well of Ascension or The Hero of Ages. Did I miss something? Or were those men Alendi and Rashek?
It's strongly implied that the Eleventh Metal works just like gold, and that both images are could-have-been visions of Rashek.
The "merchant" vision is the confusing one, naturally. It likely represents a Rashek that came out of the mountains and joined Alendi's wider society, and with his driven will, enjoyed success but possibly not happiness. The vision of him with Terris heritage embraced is much clearer, and notably happier; perhaps he lost his anger and prejudice at Alendi on this path, stayed a mountain guide, just lived as he knew. Regardless, both are what he might have become had he not taken Alendi's place and made himself Lord Ruler.
Confusion is compounded however, as when Vin uses the metal a second time, only one image is mentioned. There's also plenty of information in later books that highlight everyone's incomplete information about Allomancy, so it's possible this is incorrect. Vin herself is misinterpreting what's going on in front of her (thinking she can stab the "past" version of Rashek was not her brightest idea). The Eleventh Metal is eventually named malatium, an alloy of gold and atium. Being atium mixed, it is not a paired metal to gold like Vin thinks it is in this book, and since only pure atium is well understood, the merchant may well be a different figure altogether.
While it is true that the in the novel Vin assumes that it is an external version of Gold we are told later on in the series rather explicitly that this is not true. In fact, Gold and Atium are strange pairing in a number of different respects, not the least of which is that one uses the base metal to see the past while the other uses the base metal to see the future.
In fact, when Vin burns Malatium and looks at the Lord Ruler, what we see is the following.
And then she saw him. Another man, standing right beside the Lord Ruler. Where had he come from?
Initially, she sees only one man. If we assume that Malatium works like Atium, then this makes sense. Much as an individual only has one future when Atium is burned, an individual only has one past.
So, per your question that leaves the concern of where the second Malatium shadow enters into it? If we look forward a couple paragraphs we see the context for this second figure. (I've added emphasis based on how I read and interpreted it.)
Vin turned to the side. There was another unfamiliar man beside her, a young nobleman.
Vin has turned to the side. Why would she need to do this before seeing a second image of the Lord Ruler and why would it appear next to her and not him? So, it doesn't make much sense that this second Malatium shadow is of the Lord Ruler. So why would she see a second Malatium shadow at all? Consider that when one burns Atium it allows one to see all the Atium shadows in one's field of vision (including the Atium shadows of people one is not looking at directly, as evidenced during various fight scenes in the later books).
So if not the Lord Ruler, who else could cast that Malatium shadow? Who would be close enough to cast a shadow beside her but not be directly in her field of vision? If we back up a few paragraphs we get an answer to that to.
The Inquisitor pinned her arms to her sides, however, and backward kicking did very little good.
There is an Inquisitor standing directly behind her. If he cast a Malatium shadow it would appear beside him which is beside and behind her. So if she turns her head in the proper direction, she would be able to see it even if she could not see the Inquisitor himself and it would for all reasonable purposes be described as "beside" her.
This would neatly explain why later on, she only sees one Malatium shadow. When she uses the metal against the Lord Ruler the second time, they are initially alone. This, of course, brings a new question into play. Why would Sanderson include such a detail if it wasn't meaningful to the book? Well, if you look at the way he writes, especially during the last book, in many ways his foreshadowings act much like clues in a mystery novel, slowly guiding us toward the conclusion. By giving out this tidbit about the Inquisitor's past, Sanderson clues us in to the Inquisitors' human origins before they are explicitly revealed.