How do I back up my current 10.9.5 hard drive so that I can then create two bootable partitions - one with 10.9.5 (for legacy software) and one with 10.14

You say that you have legacy hardware on your current HD. Do you mean to say legacy software? You seem to be asking several questions without being fully clear as to what you want to accomplish. Here are my suggestions. Do you have an external drive? if so, what is it being used for? If not, then you need to get one to be used for your backups. Cloning your hard drive will provide a copy of all your HD files as well as a second operating system (10.9.5), which is good to have. If you also wish to have another operating system, 10.14.6, then I would download and install it on a separate partition of the external drive. So, according to what I've suggested, the following steps would be:

1. Use or purchase the external disk.
2. Format it into two partitions.
3. On one partition clone (copy) all of your hard drive including the 10.9.5 operating system.
4. On the other partition , download and install the 10.14.6 operating system.

Then you can select the 10.14.6 OS as a startup OS any time you need to. I hope this addresses what you were wanting to do. If not, then please reply with clarification.