People have a dual boot setup for various reasons and you could have Windows XP and Windows 7 on separate partitions of a disk drive or even separate drives. At boot time you can choose which Windows you want to start.
For many years I ran a Windows XP and Vista dual-boot setup (it still works, but is gathering dust in a corner), but these days you might be running Windows 7 and 10. Sometimes there is software that only works on an older version of Windows, or sometimes you aren't quite sure about a new version, like Windows 10.
Ideally, you want your software to be available to both versions of Windows no matter which you boot up. The question is, do you install software twice? Once on each partition so you can access it from both copies of Windows?
Do you install certain software on just your favorite version of Windows? This is a problem if you are currently using the other copy of Windows without the software you want to use.
It is a problem if you run email software like Outlook or Thunderbird and your emails will be available on one version of Windows and not the other.
Email clients are used much less than they used to be, but some people still use them. Outlook remains popular and some people use Thunderbird.
If your email provider allows it, you should access your email using a web browser. For example, Gmail, Outlook.com and Yahoo! Mail can all be accessed through a web browser. No matter which version of Windows you boot up, or even which computer you use, your email is always available.
outlook.com web mail
If you prefer to use an email client, select the IMAP option when setting up your email account if this is available. IMAP keeps your email on the server rather than on the computer. Mobile phones use IMAP to access email rather than downloading it and even if you read a message on your phone, you can still read it on the computer, tablet or anywhere else.
IMAP leaves a copy of the messages on the server so they can be read elsewhere.
Web browser solutions
The main problem with using web browsers on different versions of Windows is with bookmarks. You want bookmarks synced across the Windows versions or even across computers if you use more than one.
Edge is no use because it is Windows 10 only, and Internet Explorer is outdated now.
Chrome and Firefox can sync your bookmarks and extensions too. Whatever you bookmark and whatever extension you install in Chrome in one version of Windows or on one computer, is automatically added to any other copies of Chrome you use.
If you want to use the same software on two versions of Windows on your dual boot PC, one option is to use web-based software. Use Microsoft Office Web Apps at onedrive.com. There is Word, Excel, PowerPoint, OneNote, Calendar and other apps.
The web apps are not as powerful as the desktop Microsoft Office software, but it might be good enough for what you need. You could even install Office on one version of Windows on one partition and save your documents to the OneDrive folder. On the other partition you could use the web apps to access those files.
Office web apps at onedrive.com
Instead of online software, you should seek out portable software. This is software that does not need installing, apart from extracting it to a folder after downloading it.
Portable software runs from a folder and that folder can be placed anywhere. It could be on a disk, a USB drive, a thumb drive and so on. It can be run by different versions of Windows, so no matter which partition you boot from, you could run a portable app wherever the folder is stored.
If you explore the website of some software developers, you might find a portable version of an app that can be used instead of the regular download with the installer.
There are whole websites dedicated to providing portable versions of software and one of the best known of these is PortableApps.
Use as many portable apps as you can because it simplifies running software across two versions of Windows. There are dozens of portable apps, including LibreOffice, GIMP, VLC, Thunderbird, to mention just a few.