Jesse Smith of DistroWatch discusses dual-booting FreeBSD and Ubuntu in this brief walkthrough. There are many reasons why one would want to use both operating systems on the same computer as they serve different tasks. See the guide below for the full set of instructions.
Exploring-two-operating-systems asks: I think a good topic for your Q&A section would be dual-booting BSDs (if you need a specific one FreeBSD would likely be best) and Linux. It’s not something that is well documented on-line and many Linux users are at least somewhat interested in trying out BSDs.
DistroWatch answers: Most of the process of setting up a dual boot environment is fairly straight forward. The only tricky part, in my experience, has been in the initial planning phase. When you want to dual boot it is important to plan out how many partitions you will need, how big they will be and what will be put on each one.
For instance, if I want to dual boot Ubuntu and FreeBSD I will need at least two disk partitions (one for each operating system). I may also want swap space, perhaps a separate /home partition for Ubuntu (FreeBSD will probably keep a separate /home mount point inside its own partition). I may then also want a separate data partition where I can dump files to be transferred between the two operating systems. In the end, I may end up with a need for anywhere from two to five partitions for these two operating systems.