Second number on iMessage and Facetime

The following is extracted from Using Dual SIM with an eSIM - Apple Support

Send messages with iMessage and SMS/MMS

You can use iMessage or SMS/MMS to send messages with either phone number.* You can switch phone numbers before you send an iMessage or SMS/MMS message. Here's how:

1. Open Messages.
2. Tap the New button, in the upper right corner of the screen.
3. Enter your contact's name.
4. Tap the current phone number.
5. Tap the number you want to use.

* Additional fees may apply. Check with your carrier.

iMessage is activated only for one number at a time. Each time you change the number your carrier may change for change over and activation. See my first post * Additional fees may apply. Check with your carrier.

However

You can have multiple Caller IDs for FaceTime on your device with FaceTime.

This can be achieved by changing the Caller ID as shown in the GIF below. (Made on iPhone 6 iOS 12.4.5)

Change the Caller ID before making a FaceTime call. Enjoy privacy...

Caller ID is same as Reachable At IDs

In Order to add more Reachable At IDs (Refer to Picture at the bottom), Log into https://appleid.apple.com with your Apple ID credentials. Under Account Section Click Edit and Add as many as Reachable At Phone Number or Email IDs

GIF for changing Caller ID...

Add more Reachable At in Apple ID