I would expect Photoshop to only use the smaller F disk if the larger D disk was filled up. Is D full?
If you're on Windows you can go to Preferences and select not only the drives you want to use as scratch, but also the order in which they will be used.
As you can see in my set-up below, my "Z" drive is the first disk I want Photoshop to use because it's an SSD, and if it fills that up with temp files - it hasn't yet! - it can then use the "G" drive which is a regular 7200rpm spinner.
Hope this helps
but i want to use two hardisk togather for the higher speed even if the first driver is not full.
Okay! So you want to RAID your Scratch Disks! I've not tried that, and I don't know how Photoshop would handle a RAID Scratch Disk; I'm afraid someone smarter than me will have to visit the thread.
But even it were possible, both disks would have to be the the same size/capacity, speed, and cache; you can't RAID different sized disks like the ones you have, so you'd either have to have 2x 500GB, or 2x 160GB drives to start with, plus a dedicated RAID card to do the processing for you, you wouldn't want your CPU to be handling that load as well as everything else, the drives would also need to be internal as well, not an external RAID enclosure, where the I/O port becomes a bottleneck.
If you can, change the name of your question, "I have 2 hardisk, so I want to set two scratch disk but it doesn't work," to something like, "Need to RAID Photoshop Scratch Disks!" and hopefully someone who knows a LOT more about Photoshop and RAID than me will be able to give you an answer.
Also, please give as much information about your system as possible, what Operating System are you running, is it 32 or 64 Bit, Processor type and speed, installed RAM, Motherboard (some support RAID natively) etc.
Also, you do not want to use an email address as your user name.