mine is separate limit because the bank entity also separate

Maybank Card Centre won't give you another new CL of RM18000.

You cannot convert Islamic CL to Conventional CL.
(Conversion from Conventional to Islamic is possible).
You need to apply for a new CL from Maybank (Conventional).

If you don't offer to reduce/restructure (eg from RM18000 to RM10000+RM8000 any combination) your existing CL, unlikely Maybank will grant you a new CL for Maybank (Conventional) CC. Unless your income has been improved substantially since you got Islamic CL.

Discuss with Card Centre.

Then you secure a NEW credit line of RM18000 from Malayan Banking Berhad.

You already have a credit line of RM18000 from Maybank Islamic Berhad.

You can check the available credit limits at M2U.

You don't qualify for Maybank Islamic's new credit line, as you still had a credit line from Maybank.
Normally Maybank Card Centre won't give a new credit line, unless your income is substantially improved from last review.

Also, apply directly at Maybank Card Centre, avoid third party and Maybank2U online application.