...Static and Dynamic...
static:
Term used to describe a character whose personality and/or views do not vary/change throughout the length of the story.
dynamic:
Term used to describe a character whose personality and/or veiws change/vary throughout the length of the story.
Vash and Nicholas are dynamic characters, their beliefs change over the course of the show; but, on the other hand, Milly and Meryl are static characters, and remain the same throughout the show. Now, it's really difficult to place a static character with a dynamic character, because typically, after the dynamic character has grown throughout the story, they are different from the way they were in the beginning of the story. However, it appears that there are hints throughout the show of Milly and Meryl liking Vash and Nicholas in a more than friendly way, but I believe that this just won't work and I'm going to try and explain why.
Let's start with Vash and Meryl. Vash in the beginning is wandering trying to find his brother, Knives. It really isn't until after Nicholas's death and the killing of Legato that Vash slowly realizes that going strictly by Rem's beliefs won't work, and that he must find his own path in life. It is also thanks to Meryl when she protected Vash from the crazed townspeople, that made Vash realize just exactly what Rem meant by 'take care of Knives'. Thus making him different from the Vash he was in the beginning. Meryl on the other hand is exactly the same as she was in the beginning, just a little more accepting of who Vash truly is, and that change is not significant enough to say that she's a dynamic character.
Now, for Milly and Nicholas. Nicholas has to be the most dynamic character in the show. He starts out closed minded and cynical only doing what he was told to do. However, thanks to Vash*, he slowly starts to evolve over time and change the way he views the world. He begins to think that maybe mankind and the planet can be a better place. Milly stays a spacey airhead the entire time. So because the girls were meant to be cross matched (Milly is a female Vash and Meryl is a female Nicholas) the relationships just won't work. Or at least I (as a writer) don't think so. All because Nicholas and Vash are different people in the end, and the feelings in the girls began at the beginning of the show. Now it can be said that the men weren't ready in the beginning to accept the girls' love, but they are in the end, but I really don't think that it would work out, but then again, we never see any of that since Trigun is a shonen anime.
(added March 8, 2002)Vash and Meryl wouldn't be able to live together because of the fact that Vash is not suddenly going to start being serious all the time just because Meryl thinks that he should behave more like an adult. Milly and Nicholas couldn't live together romantically because I think that he sees her as a younger sister. Plus, the girls do not really act like crossed matched personalities. Milly is a true airhead--it's not an act, and Meryl is not sensitive like Nicholas is.
*Not Milly, Nicholas admits that Vash is the one who started his thoughts to change in episode 11