My guess is that there was no need to command, because they all understood it and many many practiced it.
However, if you want commands, here ya go. Just 22 verses after "Thou Shalt not commit adultery", we find this little tidbit in the Law, about how to treat a second wife as to her rights of Marriage:
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Exodus 21:10-11
10 If he take him another wife; her food, her raiment, and her duty of marriage, shall he not diminish.
11 And if he do not these three unto her, then shall she go out free without money.
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And later, in Deuteronomy, it tells what to do with offspring of a current second wife that is not as loved as the first wife:
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Deuteronomy 21
15 ¶ If a man have two wives, one beloved, and another hated, and they have born him children, both the beloved and the hated; and if the firstborn son be hers that was hated:
16 Then it shall be, when he maketh his sons to inherit that which he hath, that he may not make the son of the beloved firstborn before the son of the hated, which is indeed the firstborn:
17 But he shall acknowledge the son of the hated for the firstborn, by giving him a double portion of all that he hath: for he is the beginning of his strength; the right of the firstborn is his.
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Sure is odd that for a people that were not supposed to have multiple wives, there is mosaic law regarding day to day details and problems that arise from such relationships.
At least you admit you are guessing.