What I could contribute about this layout:
Suppose that you have two separate Operating Systems (Windows & Ubuntu), and assuming that one of the operating systems grabs malware, such as spam / advertisements / apps that are spamware / etc, then it is very unlikely that this type of malware will spread to the other operating system, no matter where the other operating system is. It could even be on the same drive. Since this type of malware is very unlikely to support both operating system, then it will only infect one operating system. So you can have Windows & Ubuntu on the same drive, or different drives.
YET, there is one condition about malware. Assuming this malware that you grab is a very terrible type of malware (not spam nor advertisements), and is some sort of Trojan or Virus or such, where a person can take control of your computer, then no matter what, everything that is currently connected to the computer may be at risk, depending on what the person wants to do with your data that this person has access to. They could be very harmful and lock all your files until you pay them money/etc, but it all depends. There are also some malware that could just erase everything that's on the computer.
But there's nothing to worry about these malicious types of malware,
unless you open an untusted link, or some sort of advertisement stating that your computer is at risk, call this number, etc. If you ever see some weird "unprofessional" message pop up on your screen, it's most likely an advertisement from your internet browser that popped up from some website, and you shouldn't be tricked by it. Just close those types of windows.
Here are examples of pictures of what you shouldn't be tricked by.
Those are only example images from a search engine:
warning computer infected - Bing images