How to execute dependent script library when executing main project using TestRunner in command line

Thumb rule, every test should be independent. But it is talking about dependency on two different project, not good / viable for test automation.

So you can convert them into library with a jar file and copy under SOAPUI_HOME/bin/ext directory.
To do so, please refer our expert's blog by rupert_anderson
http://rupertanderson.com/blog/1-how-to-develop-add-and-use-a-custom-groovy-library-in-soapui/