Can I have 2 accounts on my iPhone???

Just to add, if you want to get your emails belonging to the old Apple ID you will need at least one Apple device or a trusted number set up on https://appleid.apple.com on which to approve 2 factor authentication when adding your old Apple account to the Mail app.

Longer term you would do better to ask all your correspondents usinhg the old Apple account to use your new account.

Follow-up question, why was it necessary to create a new Apple account for the new phone?

See this Apple article: Sign in with your Apple ID

Sign in with your Apple ID - Apple Support

👉 Your Apple ID is the account that you use to access all Apple services and make all of your devices work together seamlessly.

You can't merge multiple Apple ID.

I want to use my old Apple account as well for the emails

If you have not deleted your old Apple account, you can still use it.

& yes, you can use 2 accounts on your iPhone.

See also -> Add and manage email aliases for iCloud Mail on iCloud.com

Add and manage email aliases for iCloud Mail on iCloud.com – Apple Support (UK)