OP:
You said in post 1 that both drives total up to about (only) 100gbs, is that right?
Don't waste your time worrying about this.
Get a 128gb 2.5" SATA SSD. These can be bought for $20.
Get an enclosure like this:
SABRENT 2.5 Inch SATA to USB 3.0 Tool Free Clear External Hard Drive Enclosure [Optimized for SSD, Supports UASP SATA III] (EC-OCUB)
www.amazon.com
Put the drive in the enclosure
Erase it to HFS+ (Mac OS extended, journaling enabled, GUID partition format)
Copy the data from each drive to it.
Then... retire the two old drives.
Problem...
solved.