Combining two premium managed disks or one separate Disk is good to match capacity

I have this issue from lot longer, when purchasing Azure managed disks I have a requirement of allocating 512GB premium disk. I'm wondering if I allocate two P15 (256GB) premium disk will also give the same as P20 capacity with small pricing different, IOPS and throughput. I need to answer the following questions :

1. Which approach is best to match 512GB is it allocating single (P20)512GB or two 256GB (P15) disk ?
2. If I allocate two 256GB (P15) disks will that double the IOPS and Throughput ?

From the managed disk price, two 256GiB (P15) is a larger price than one P20 512GiB premium disk. Also, the two P15 disks have 1100*2 less than P20 disk 2300 IOPS but have double throughput.

Take into account considerations listed below when making the choice.

Scale Limits (IOPS and Throughput) The IOPS and Throughput limits of each Premium disk size is different and independent from the VM scale limits. Make sure that the total IOPS and Throughput from the disks are within scale limits of the chosen VM size.

For example, if an application requirement is a maximum of 250 MB/sec Throughput and you are using a DS4 VM with a single P30 disk. The DS4 VM can give up to 256 MB/sec Throughput. However, a single P30 disk has Throughput limit of 200 MB/sec. Consequently, the application will be constrained at 200 MB/sec due to the disk limit. To overcome this limit, provision more than one data disks to the VM or resize your disks to P40 or P50.

So If you have a high scale VM, your application requires a larger throughput and it supports write or operate data to both disks at the same time to optimize the two disks' throughput. You could select two P15 disks, otherwise, generally, it prefers to use single P20 than two P15 disks.

For more information, you can see Azure premium storage: design for high performance.