Can I have the same phone number on two iPhones with different carriers?

No. But if both phones are on the same Apple Account, you can use Calls on Other Devices from the phone with the number you'd like to have ring on the other phone. Settings > Apps > Phone > Calls on Other Devices and choose the other phone.

This is fundamentally a carrier question.

Ask your preferred carrier if they have a configurable call roll-over function, where you can set a second number to call if the first phone doesn’t answer within a few rings.

If they don’t, ask them what other options they might offer.

They might be able to group together numbers for instance, if you use one carrier for all numbers. This was once called distinctive ring or a party line. This happening across two carriers, not so much.

As for two different carriers both with one number, nope, definitely not.

Most carriers offer Call Forwarding. I use it occasionally when traveling. I forward business calls to personal iPhone. You turn it on and off from your device.