I have two hard drivers in a NAS, and I cannot mount both at same time.
If I mount first one, second try to mount using the same folder as the first.
I can see there is a race condition there.
I've had this problem, the closest thing to a "fix" (quotes because it doesn't work all the time) I've found is to mount one disk through Luci > Mount Points and the other through rc.local (Startup in Luci) after a delay. So, for example, you have Drive 1 that you mount through Luci > Mount Points and for Drive 2 you'll mount in Luci > Startup with this -
sleep 5
mount /dev/sd* /mnt/drive_2 -o your,options
And then reboot and see what happens. If ti doesn't work, check to see what the /dev/sd* of the drives are and reverse the /sd* (meaning if drive 1 was /sda and drive 2 /sdb just make drive 1 /sdb and drive 2 /sda) and reboot again to see if that works. What's happening is the /sd* is getting changed for some reason. This occasionally works for me, but not all the time.
I just remembered this, I meant to look into it to see if it applies to this issue but never did, just sending it your way in case you want to give it a try.