Rashi on Numbers 10:3/רש"י על במדבר י׳:ג׳

1ותקעו בהן WHEN THEY BLOW WITH THEM — with both of them; and this (protracted sound, a “Tekiah” on both trumpets) is the signal for summoning the congregation, as it is said here, “then all the congregation shall assemble unto thee at the entrance of the tent of meeting”.