For us, it didn't matter which we paid first. Since AoS was first on the screen and IV was beneath, I just paid the AoS bill, then when the screen refreshed, I paid the IV bill. Receipts were emailed to my wife and I.
The payments showed as pending in my bank account, and in process on CEAC. I think it was about 3 business days later or so the payments showed "paid" in CEAC. Basically, they cleared from my pending payments in my bank account, and then next day, they showed "paid" in CEAC. We didn't get any emails or notifications from CEAC about the payments posting. But when I checked CEAC, they showed paid, and the links for AoS and the DS-261 were live, so we could start working on them and submitting documents.
I already had the I-864 completed and all my documents saved in the cloud so I could just upload everything.
Then, when we had time, my wife and I video chatted and screen shared, and we completed her 261 and uploaded all her documentation.
Just be aware, at least for us, CEAC crashed repeatedly. (I believe the servers overloaded and we got booted, actually.) There's a thread or probably two in here somewhere of me ranting about CEAC.... 😬🤦🏽♂️
I hope it all goes smoothly for you both. 🤙🏾