Is it היו or יהיו פני הכרובים?

Summary: The Rashba answers a query, based on the texts before him and bolstered by sevarah.

Post: Similar phrases occur in parashat Terumah and parashat Vayakhel:

שמות פרק כה

• פסוק כ: וְהָיוּ הַכְּרֻבִים פֹּרְשֵׂי כְנָפַיִם לְמַעְלָה, סֹכְכִים בְּכַנְפֵיהֶם עַל-הַכַּפֹּרֶת, וּפְנֵיהֶם, אִישׁ אֶל-אָחִיו; אֶל-הַכַּפֹּרֶת--יִהְיוּ, פְּנֵי הַכְּרֻבִים. 

שמות פרק לז

• פסוק ט: וַיִּהְיוּ הַכְּרֻבִים פֹּרְשֵׂי כְנָפַיִם לְמַעְלָה, סֹכְכִים בְּכַנְפֵיהֶם עַל-הַכַּפֹּרֶת, וּפְנֵיהֶם, אִישׁ אֶל-אָחִיו; אֶל-הַכַּפֹּרֶת--הָיוּ, פְּנֵי הַכְּרֻבִים. 

Minchas Shai reports an interesting shayla which came before the Rashba:

"היו פני -- The Rashba wrote: You have written to be that there are in {your} sifrei Torah both in parashat Terumah and in parashat Vayakhel the words יהיו פני הכרובים, and in a few sefarim are written in parashat Terumah יהיו and in parashat Vayahkel היו. Such is written in our sefarim -- in parashat Terumah, which is a command, is written יהיו and in parshat Vayakhel, which pertains to the actual performance, is written היו." 

This accords with our own sefarim. Besides the testimony and explanation, I would note that the Samaritan text also accords with our (and the Rashba's) text, and Vetus Testamentum does not record any Jewish variants. We might have expected Samaritan scribes to harmonize, but they don't. If anything, they harmonize out the ויהיו in the beginning of the verse to match the היו at the end. So perhaps this can be taken as further evidence, as it goes against the grain of Samaritan emendation.