Seeing as the Mishkan comprised two levels of Kedushah (the Kodesh and the Kodesh Kodshim), why does the Torah write "Vehayah ha'Mishkan Echad"?
Seforno: Because the two sections worked in unison to perform the will of Hashem. 1
Rashbam: With reference to the bottom set of curtains, the Torah is simply saying that the two sets of five curtains became one via the series of loops and hooks. 2