I have a Lincoln and a Ford but still get separate billing.
While I hate to continue the Sirius XM billing sub discussion, you should have no problem combining bills.
As soon as they started combining lineups and later on billing, I was able to combine my accounts with ease. I had two subscriptions of each since very early on in each companies respective existence. Sirius at both home and in the car and XM at both home and in the car with plug and play units. And then I went down to one subscription of each on a combined bill. XM at home since it was built into my (new at the time) Sony A/V Receiver and Sirius in my car on my Navigator. Then later I had an Expedition on there.
So it was the following on one bill at one point:
2013 Navigator – Sirius
2014 Expedition – Sirius
Sony AVR – XM
The 2014 Expedition was traded for a 2019 and never activated. My Navigator was traded for my 2023 Bronco 4 door which I had for a whole 5 months before I got rid of it and I upgraded the AVR and the new one doesn’t have built in SiriusXM. The Bronco with Sync 4 had all of the Extra channels, basically everything in the app was on the car radio, but it was using the built in cellular modem obviously, which was powered by AT&T, so if there was no or poor AT&T coverage you just had the normal satellite channels.
I replaced the Bronco with a 2021 Expedition Limited that was upgraded to where it is pretty much a Platinum minus the massaging seats, 22 speaker audio system and fancier front grille. But it has Sync 3, the 2022 mid-cycle refresh got the 15” touch screen and Sync 4 and the upgraded SXM experience. I can tune to up to channel 460 on the Expedition.
What was cool was I got the free three month for each the Bronco and 2021 Expedition and then 3 more months for a $1. So I got 1 year of service for $2 and I only had to by $120K worth of SUVs to get that deal
If you are having trouble combining accounts, I wonder if one car has a Sirius radio and one is SiriusXM and they are having problems combining them because of that. I'm not 100% sure and I could be wrong, but I thought it loosely corelated to the infotainment system. Sync 1 and Sync 2 were Sirius only and Sync 3 and 4 are SiriusXM. If both of your cars are on the same version of Ford Sync that is really baffling why they can't combine them.