Whether a registered person will have to approach two authorities – Centre as well as State for various permissions, audit etc. under the Act?

No, a registered person will have to approach only
one tax authority for all practical purposes. Each registered
person would have one tax administration office, either of
the Centre or of the State. Legal provisions (called crossempowerment)
have been made to ensure that one officer
can discharge all functions under CGST, SGST and IGST Act.
The registered person would be informed of the tax administration
concerned with him. A single registration is granted
for the purposes of CGST, SGST/UTGST and IGST.