I multiboot like many here. There are a few considerations when adding more
partitions and new distros.

When in a new distro is installed it can have unexpected consequences no matter
what precautions you take. As a minimum you need to be marginally aware of the
boot process, grub editing and be risk tolerant.

When you install a new distro you need to have a place to put it which means
that you need to have a big drive and be willing to resize and create new
partitions or add other drives. Two drives is better for many reasons. When you
resize a partition it may cause you problems when you re-boot. Sometimes it will
cause fsck errors due to the fact that you have altered the drive which is
journalled and may get consistency errors or problems with the file table. In
which case you will get a simple hiccough in the boot process and need to press
Ctrl-D at some point. Something that you have done has created a mismatch in the
file records and this just causes it to pause and give you an opportunity to
repair it or not. The ctrl-D will continue each time till you repair it or till
you re-do the file system by re-installing.

When you add a new distro it will want to add to the boot loader which will
replace the old one. If you have two drives you can have separate bootloaders on
each drive. It can get very complicated as grub uses relative disk labeling
which depends on which drive the distro is installed to rather than the physical
cable selection as determined by BIOS. For example if you have sda1, 2, 3 etc.
and hda1, 2, 3 etc. as I do and install a new distro to hda4 it will want to
install the bootloader to hda1, even though sda is the primary drive. This will
write grub to hda with hda1 becoming hd0,0 in grub, but if you install boot sda
first by BIOS then the bootloader won't even load and if it does then it may not
work. All very confusing to say the least. You need a very logical problem
solving mind to sort it all out.

I don't want to steer you off, but just be forewarned that anything can happen.
You need to know what you have installed to which drive or partition at the very
least such as Windows is on sda1, Ubuntu us on sda 4, /home is on sda 2 and
Fedora is on sda5. The same for hda drives.

I keep a list as I typically have half a dozen distros installed on the same
computer at any given time a d the landscape is constantly changing. I install
one or two a week. Obviously I have had problems and have had my share of
failures.

If you want to be secure use separate drives. Keep one drive for your primary OS
and make it the default boot device and do not write to it ever. This means that
you must pay close attention of where grub is written to. If your main OS is on
sda then always write grub to hda. If you are unsure then don't proceed. Most
distros tell you where it will be written to and give the option to change it.
If it does write to sda by mistake be thenkful that it is grub and not the
Windows bootloader as it will likely include Windows or whatever your main OS is
in the lsit of choices.

The odd time a bootloader will not pick up all installed distros and some are
better than others at picking up everything. In these rare cases you may have to
manually edit grub

It is not only possible, but it also a lot of fun. Just be sensible and go
slowly isolating things most important to you, back up if you are worried and
always expect the worst and be thankful when it does not happen.