Sena or Cardo work in 2.4 Ghz (ISM) range as well, but as a class 1 devices, which with 100mW (20 dBm), they can allow for maximum range in excess of 1 mile.
I'd use Walkie - talkies (PMR 446 MHz, about half of a mile of the range in the town) before resorting to the smartphone bluetooth. Likely only feasible on the parking.
Smartphone bluetooth is fine for two bicycles but it does NOT compete with a purpose-built hardware, especially not with the top makers like Cardo or Sena, LOL.