Can I Move Content Between Two Adobe Stock Accounts?

NO! You cannot upload or move your brother's content to your account. You must be the owner of all content on your account.

Yes, absolutely. Read your Adobe Stock Contributor Agreement. You are not permitted to upload content to your content that you did not create yourself. Or is your brother's account really just one of your accounts?

So the assets you want him to move are his assets? No, since you don't own the copyright to those assets. Your account should only contain your assets and his account his assets.