Best setup for two internal HDDs?

I'm about to configure a new PC, a Lenovo K430. Annoyingly, and contrary to what their tech people first told me, it has bays but no connections for an additional internal HDD and second optical drive. So I'm planning to disconnect the USM SATA and power cables, and run them to a second internal hard drive.

before doing that, I'd appreciate advice. In my current desktop, I have two internal drives configured like this (with help from Easus Partition Master):

Drive 1

C: partition (25 GB) has Windows, and all the "My Documents"-type stuff that Win seems to want to keep on C:.

Rest of drive is one big partition that receives auto-backups (via Cobian) of all data from Drive 2.

Drive 2

Small partition for Windows page file.

Main partition for all other user data plus programs.

This setup seems to work very well, and makes boot-up and operation far faster than with the original one drive.

However, as may already be obvious, I don't really know what I'm doing here. So, two questions:

1. Is there a better way to configure two drives?

2. If I just use a USM HDD for second drive, which items should be stored there? My understanding is that this will be a slower drive than a regular internal HDD, but perhaps that's incorrect.

All advice appreciated. Apologies in advance if this is a silly question.