Can I use same domain user "DOMAIN\USER1" for both MTSUser and Default SDL Web Administrator during CMS Database installation. Reason being I only have one domain user which has admin rights and i am doing SDL installation using this user only. Is there anything wrong if I use this user for both MTSUser and Default SDL Web Administrator ?
As Nuno has already commented that you can definitely use the same user for both MTS User and Default Administrator. Further, this is not mandatory to have your MTS User to have Admin rights.
You may refer below blog post of mine which mentions what rights are required for MTS users instead of being a Domain administrator: MTS User Minimum Access Rights