Hey, nice idea. Now I kind of wish I had gotten that HDD + SSD upgrade when buying that laptop..
It's not too late. Depending on your model you have a couple of options:
1) Obtain a 2nd HD caddy to replace your optical drive (assuming you have one) with an SATA HD bay. Most modern laptops take a
9.5mm, some (mostly older models) take a
12.7mm one. I've done so many that I can pretty much tell which by just looking at it. If in doubt you can always
find one on ebay and ask the seller if it will fit your model.
You simply remove the optical drive, CAREFULLY detach the escutcheon/cover from it, put it on the caddy, install a 2.5" drive in it, then reinsert it into the laptop. You can skip the escutcheon/cover part if appearance isn't an issue. Locate your laptop repair/disassembly guide/service manual for guidance if it seems a bit daunting. Should you ever need an optical drive (rarely I'll bet) you can put yours into a
USB enclosure and simply plug it in when needed.
2) Alternatively, if your laptop supports booting from USB (nearly all modern ones do), you can install an OS to a secondary USB HD or flash drive (USB 3.0 is preferred). Won't be nearly as fast as native SATA, but it works well enough for my needs.
I installed (vs running live) Lubuntu to a 64GB USB 3.0 flash drive on my i5 Lenovo with 16GB RAM, and can hardly tell the difference. I actually just did an in place upgrade of Lubuntu several weeks ago w/o issue and it didn't take long at all. This would be
the most economical method if speed isn't of great importance, and there's no need to modify anything. You can still mount your Windows partitions to access or place larger files on.
I haven't done this the other way around yet (native Linux w/Windows to go), but it should work the same. Just don't let Windows touch your primary disk. Pull it out if in doubt.