Nice one - always good when something works out well!
If you do want to add a 2nd HDD, be aware of a few things - you will probably struggle to find a disk < 2GB, which is the maximum size that DOS 6 and Windows 3.1 can recognise, using FAT 16. The BIOS may not even be able to recognise disks over 540MB, but you might be lucky with a later BIOS that can recognise disks up to 8GB, using various techniques, such as LBA. You would have to create multiple partitions each less than < 2GB for DOS to use such a large disk.
Assuming you do find such a disk, or have one spare, you will need to ensure that you configure the jumper settings correctly on the disks - existing HDD should be set to Primary and the new HDD to Secondary. We used to say Master and Slave, but that's not PC (pun intended) any more. If your IDE cable supports it (80-wire with 40-pin connector Ultra ATA cable) you can set the jumpers on both HDDs to CS (Cable Select), with the current boot disk on the Primary connector (should be written on the connector). CS is supposed to be better, due to less noise on the cable.
At least now you know how to set the disk parameters - just use Autotype!
SD