Practical answer
Can I keep the same number on two phones?
On a supported shared-number service, yes. Through plain call forwarding, partly — calls follow, texts usually don't.
There are two different things people mean by this, and they behave very differently.
Shared number service
A provider links a second, simpler device to your existing number. Calls and texts reach whichever device you're carrying. This is the smoothest option and the reason the One Number. Two Phones. setup works. It depends on your carrier and both devices being supported, and it costs money every month.
Call forwarding
Your carrier forwards incoming calls from one line to another. It's cheap and available almost everywhere. But it generally forwards calls only — text messages stay on the line they were sent to, which surprises people.
Before buying hardware, confirm compatibility with the provider. Support varies by carrier and device and changes over time.