In my opinion, I'd try and get as close to both as possible. I would hate to sacrifice quality sound on one person for BG or balance, just because the other is farther away from the mic. If only one character is sitting down, that character will probably be looking up at the other character, that could help. Same with a tall person situation. I've been taught to give the best recording possible for every single line of dialogue. You can always hope for coverage. And if you don't get it, most post people could easily level the two out if they needed to.
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In my opinion, I'd try and get as close to both as possible. I would hate to sacrifice quality sound on one person for BG or balance, just because the other is farther away from the mic. If only one character is sitting down, that character will probably be looking up at the other character, that could help. Same with a tall person situation. I've been taught to give the best recording possible for every single line of dialogue. You can always hope for coverage. And if you don't get it, most post people could easily level the two out if they needed to.