The ACARS uses VHF COM3 for it's data bursts. It does not need to be tuned to any freq on the control head. VHF COM3 is useable while ACARS operates 'in the background', but will not ACARS XMIT or ACARS RECIEVE while you are voice XMITTING on whatever freq you have dialed in the control head (or FMS COM page).
You can sometimes hear the screech of the ACARS burst transmit and recieve as it happens if a 'sympathetic' freq is tuned. So happens 121.5 and 123.45 are both sympathetic.
It can also use SATCOM if so equipped and enabled; same deal, is in the background, and does not interfere with voice comms thru SATCOM.