Have a read through
this thread.
Ignore the stuff about Windows. The rest of it is very relevant to what you want to do and if you look at the Partition Editor screenshot, what you want is exactly what I've done. Sunrat has almost the same, so look at his as well.
Each distribution must have its own /home and you can, if you want to, have / and /home in the same partition. I don't because I don't like having data in the operating system partition. Create a separate partition for all your data on your HDD and another on the SSD for commonly used files, if you have the space. Your /home can be quite small if you don't actually use it for data, i.e. just settings and config files. Don't forget to point things like Thunderbird to a dedicated Downloads folder in your data partition and do the same for other programs that use a default location in /home. After you install both distributions tidy up /etc/fstab in each to mount your data partition and make sure that your swap is correctly referenced.
One tip: If you're going to run Debian and Ubuntu, install Ubuntu first. It has a nasty habit of making a mess in the boot partition in my experience. I've had the same as you're planning and found that having both on the same machine really makes it obvious how much slower Ubuntu is than Debian. Slow to boot, slow to use and using sudo for everything drives me nuts. Each to his own, though. I will concede that Ubuntu does make installation with odd hardware much easier, though.
