dual-boot (ElCapitan and Mojave) on a Mac Pro 5,1?
One possible solution is to install rEFInd Boot Manager. This is free software which can be configured many different ways. Below is a suggested configuration.

By default, rEFInd installs in the EFI partition. You can then configure rEFInd to either boot El Capitan or Mojave. The default operating system to boot can be set directly from the operating system. There is simple AppleScript application you can download which will do this. As example of the application is shown below.