You can use the ‘Start Process’ activity to get one process to trigger another. Both of these processes can be built using REFramework and it won’t cause an issue. This is like the excercises on the training platform where you build a ‘loader’ process and a ‘processer’ process that both use REFramework.
That’s on the understanding that you’re talking about published processes, however. If you’re talking about using the ‘Invoke Workflow’ activity to invoke an REFramework project from within another, I don’t think that this would work, simply because the ‘Invoke Workflow’ activity is used to call a single workflow (XAML file) whereas REFramework is a project made up of multiple XAML files, plus config files etc.