Lets take common numbers:
SATA HDD - 120Mb/s read, write speed heavily depends on gaps in sequence
SATA SSD - 550Mb/s read, 500Mb/s write
PCIe SSD - 2500Mb/s read, 1500Mb/s write
Lets assume they are connected to AHCI controller (not RAID controller). If we connect two identical drives to AHCI (not RAID-0) and use two-threaded application running on two CPU cores to read/write sequental data, will be speed doubled?