Normal redundancy two failgroups

I've got a diskgroup created by

CREATE DISKGROUP FRA NORMAL REDUNDANCY

  FAILGROUP FRA1 DISK

   '\\.\ORCLDISK1FRA10','\\.\ORCLDISK1FRA20'

  FAILGROUP FRA2 DISK

   '\\.\ORCLDISK2FRA10','\\.\ORCLDISK2FRA20';

Each of 4 disks (two disks on two separate storage) is 1,5 TB size. I expected to have 3 TB available capacity in diskgroup. But It shows only 1,5 TB. Can I have 3 TB available capacity from 4 disks? How?

I'd like to be safe if one of storage is destroyed, that's why I set two failgroups.

Would it help to set

ALTER DISKGROUP FRA ADD TEMPLATE unreliable ATTRIBUTES (UNPROTECTED);

?