Another interpretation: they are called heavens (shamayim) because they make appalled (mashmim) the deeds of creatures. If people are worthy, then 'the heavens declare His righteousness' (Psalms 50:6); and if not, then 'the heavens shall reveal his iniquity' (Job 20:27).
The Heavens That Bear Witness to What People Do
Yalkut Shimoni on Torah 3:5
דָּבָר אַחֵר, שֶׁהֵן שָׁמִים מַעֲשֵׂיהֶן שֶׁל בְּרִיּוֹת, אִם זָכוּ (תהלים נ, ו) "וַיַּגִּידוּ שָׁמַיִם צִדְקוֹ" וְאִם לָאו (איוב כ, כז) "יְגַלּוּ שָׁמַיִם עֲוֹנוֹ".