If you have the paid version of MultiPSK then you have one of the only VDL-2 decoding programs available. Although it can be used with certain receivers connected to your PC, it works best with an RTL 'stick' as MultiPSK can control it's tuning etc. directly.
Regarding your question about airlines versus what provider they use for legacy ACARS, for any one operator it's going to be one or the other 99.9% of the time as the airline/aircraft operator is paying for the service. Coverage is so similar that it would be pointless paying for both. The aircraft equipment is programmed with a primary frequency to log onto the network, so that's going to be 131.550 for RC/ARINC or (I'm guessing based on recent changes) 131.725 for SITA, in North America anyway. (BTW, there are a couple of smaller regional providers in Japan and eastern Asia.)
Alternate frequencies for any particular exchange are sent by the network as a message like this:
10/02/17 01:02:09 UTC
Mode: 2 Identifier: C-FMWV Ack.: 1
Message type: :; (Uplink - Command change frequency) Block: P
Message: 130025
The radio for the data link system in the aircraft will automatically re-tune to (in this case) 130.025 MHz.
If you're close enough to an ACARS ground station, the squitter (SQ) message will tell you the provider using the frequency. Look at the 4th character in the MESSAGE field, it will be either A (RC/ARINC) or S, SITA. In this case the message is also ended with /ARINC:
10/02/17 01:07:44 UTC
Mode: 2 Identifier: Ack.: {NAK}
Message type: SQ
Block:
Message: 02XAYYZCYYZ24340N07937WV136975/ARINC
Here's a SITA squitter:
10/02/17 01:10:32 UTC
Mode: 2 Identifier: Ack.: {NAK}
Message type: SQ
Block:
Message: 02XSYYZCYYZ04340N07937WV136975/
Note that they don't bother to encode /SITA at the end.
You'll also see reference to 136975 in both of these. This automatically identifies the ground station location as also providing VDL-2 service, which uses that frequency as it's primary frequency globally and unlike legacy ACARS, at this point it's common to BOTH PROVIDERS.
A squitter from a non-VDL2 SITA ground station looks like this:
10/02/17 01:12:52 UTC
Mode: 2 Identifier: Ack.: {NAK}
Message type: SQ
Block:
Message: 00XS
And from RC/ARINC it looks like this:
10/02/17 01:21:51 UTC
Mode: 2 Identifier: Ack.: {NAK}
Message type: SQ
Block:
Message: 01XAYHMCYHM2ARINC
Notice the RC/ARINC squitter identifies the ground station, in this case YHM/CYHM, Hamilton Ontario.
Finally, the daily AZA flight from here back to FCO just started its log-on sequence to legacy ACARS seconds ago and it is with SITA on 131.725. It's still at the gate at this point.
10/02/17 01:38:48 UTC
Mode: 2 Identifier: EI-EJH Ack.: {NAK}
Message type: Q0 (Link Test) Block: 6
Message: S20AAZ065F
And then it logged onto VDL with SITA as well. (2CE757 is the identifier of the CYYZ SITA VDL-2 ground station.)
10/02/17 01:46:44 UTC
Destination address (hexa): 4CA87D (Aircraft)
Source address (hexa): 2CE757 (Ground Station, delegated address [CYYZ Toronto 43°42'N 079°36'W] , On the ground , command frame)
I (Information) frame N(R)=2 N(S)=1 P=0
ACARS message ("AOA")
Data (hexa): FFFF0132AE4549AD454AC831DF7FC2839B6A7F
Mode: 2 Identifier: EI-EJH Ack.: 1
Message type: _ (General response. Demand mode. No information to transmit) Block: B
Message:
Cheers!