Accessing files from a second partition

Diligence and discipline will be required.

Ideally, the best practice will be to keep all of your personal user documents – your designs, resources, Adobe CS files, photo libraries, music – saved and stored on a separate partition if not a separate drive altogether, or even to a cloud location. That way, no matter what OS you boot to or what user you log in to, all of your files will be available to access.

You will want to set preferences for file locations in all of your apps to point to the EHD or partition, or to your cloud location. The setup is not usually difficult, and it would make life down the road easier. You would not have to transfer files back and forth at all.

Yes, dual booting will have a separate user account in each OS. And each OS will have it's own set of applications installed.

You would probably want to base the majority of your work in the newer OS, Mojave, along with your email, contacts and calendars, though if properly setup those apps and datas will cloud sync and remain accessible regardless of which OS you were booted into.

I think 100-150GB per OS would be more than sufficient.

Since we're talking about Yosemite and Mojave, the macOS resource requirements are not great. The OS and included apps themselves can be installed with less than 40GB each (that's being conservatively high). Still, it can be hard to estimate because it will partly depend on what third party applications you need to install. You could easily install your third-party applications on the external drive as well as keeping your docs there and that would keep the drive space needed for the OS partitions to a minimum – well under 100GB each I imagine.

For example, my recently reinstalled High Sierra iMac is currently using less than 50GB on the boot drive, with the most basic of Apple softwares, a few months worth of user files and documents and little else.

You should look at what your machine is using now with Quark and Adobe and whatever else you have installed, minus the user files that you will be storing in the ext drive, plus something for expansion and make a guess from there.