This might sound strange but the effective Jewish Halacha allows for marrying multiple women at once:
נוֹשֵׂא אָדָם כַּמָּה נָשִׁים אֲפִלּוּ מֵאָה בֵּין בְּבַת אַחַת בֵּין בָּזוֹ אַחַר זוֹ - A person can marry several women simultaneously or once after another. Rambam Mishneh_Torah%2C_Marriage.14.3
To my best knowledge, Yaakov already knew his destiny of marrying the two sisters, he only wished to marry them in the opposite order. Lavan was also eager to marry off the two to Yaakov.
Marrying Rachel and Leah at once would save the hoax of switching between them and start the happy marriage from the right foot.
So why didn’t Yaakov marry both sisters simultaneously?