I just figured out a trick so that you basically shuffle numbers between these two carriers to take advantage of their deals, without ever having to rely on a third carrier to generate a new number, or worry that Xfinity might deny you will rarely ever have to rely on a carrier to generate a new number. All you need are 1) a supply of broken phones to swap to Visible, 2) a new IMEI for every BYOD phone activation on Xfinity, and 3) a Visible compatible phone with a bad IMEI (we'll get to that later).
Step 1 - Let's just assume you are starting with a disposable number from Xfinity. Order a swap phone from Visible. Do use a referral code to get $20 discount. When you order, make sure that the phone number and account number are correct. The 6 digit port-out PIN can be anything for now; it can be corrected later when you activate.
Step 2: When you get the swap phone and SIM, do port-in activation to Visible. The port-out info is easily generated on Xfinity's website.
Step 3: Once the line is activated on Visible, get on a chat with their bot and request that your phone number be changed. It can be to any area code. The bot will process this automatically for free, and the number change only takes 10-15 minutes.
Step 4: Keep using the Visible line for a month. I use these now as data SIMs on my iPhone SE 2nd gen paired with my voice line on eSIM. I've also started using Visible for my parent's home internet on a Cudy router.
Step 5: Order a BYOD SIM from Xfinity, with an IMEI you have never used on your Xfinity account. Once received in the mail, activate that SIM to port the newly generated number on Visible. Note that this number has never been on Xfinity, and you are using a new IMEI, so all should be good on Xfinity's end to get your GC.
Step 5.1. This is where the phone with bad IMEI comes in. You generally want to wait to do the Xfinity order when their BYOD gift card value is $200 (as opposed to $100 or $50). Oftentimes, this will mean that the timing on the end of your Visible billing period won't correlate. Furthermore, while Visible does allow port-out of inactive lines for as long as 60 days after service has lapsed, I have found that are often problems with porting out lapsed lines to Xfinity specifically (though not other carriers).
If the timing isn't good, there is a way to keep the number eligible for port-out indefinitely: before the Visible service goes inactive, take the SIM out of your active phone and put it in to one that has a bad IMEI. Somehow, this puts the Visible line in a state that is neither active or inactive, but can be ported out easily, even to Xfinity.
Step 6: Keep the line you've just activated on Xfinity active for the requisite 3 months to qualify for your GC.
Step 7: Once that 3-month period ends, the number is free to be ported back to Visible. If you want to truly play it safe, ask Xfinity to change the number just before porting 'back' to Visible - Xfinity also allows free number changes though you do have to call customer service.
Rinse and repeat.