The problem is with Gparted, not LMDE.
I haven't tried it, but Gparted has a drive selector in the upper right which should allow you to change drives after you have set up / and swap on drive 1 to switch to drive 2 and try to set /home on that drive.
I am assuming drive 1 is a small SSD and drive 2 is a regular rotating disk.
LMDE Mate 64-bit, LM16 Mate 64-bit
Debian Xfce 64-bit, Xubuntu 13.10 64-bit, Xubuntu Trusty Tahr 64-bit, Antergos Xfce 64-bit, PCLinuxOS LXDE 64-bit