Number not available. Which number would you like to send the message with ?

Using one Apple ID on multiple devices is not the issue here.

The issue is...

1. Both the Phone numbers must have iMessage Activated on their respective devices
2. Both Phone Numbers must have an SMS/MMS plan enabled by your Carrier to send messages to non-iOS users.

Know how to activate iMessage: Activate iMessage and FaceTime using your… - Apple Community

Using the same Apple ID on both your devices will have the following addition featured as desired by you.

You said: "hat have the same Apple ID (since I am one person) since 2018. I like having the 2 iPhones sync messages, calls, photos, notes, etc. through the Apple ID. "

On your iPhone, go to Settings > Messages > Text Message Forwarding. If you can't see Text Message Forwarding, go to Settings > Messages. Turn off iMessage, then turn it back on. Tap Send & Receive, tap Use Your Apple ID for iMessage, then sign in with the same Apple ID used on your other devices.

Forward SMS/MMS text messages from your iPhone to your ...

Do any of the following: Tap Calls on Other Devices, turn on Allow Calls on Other Devices, then choose the devices on which you'd like to make and receive calls. This allows other devices where you're signed in with the same Apple ID to make and receive calls when they're near your iPhone and connected to Wi-Fi.

Allow phone calls on your iPad and Mac - Apple Support (IN)

All these features will sync on each device when you add the email account on both both devices, Contacts, Calendars, Bookmarks, Notes, Voice Memos, shared photos, iCloud Photos, and emails. Each device syncs to the email account server which may be Google, iCloud, AOL, Exchange, Outlook, Yahoo, or any Other. Please see the pic below.

For Audiobooks, iMovie Project, and Apple Podcast log in with the same Apple ID as used in your Mac

If one of the devices is not connected to the internet then it may seem to you that they are not syncing.