Our general yuan and luo points theory page is here. For your specific question, the main answer is that it -always- matters why you are doing something for the patient. In other words, there are limited fixed rules in Chinese Medicine, the entire value of the medicine is the adaptability to the individual patient. So while LU 7 and LI 6 could be used together in certain cases, in others it would be confusing energetically or even outright wrong for others. It is not always stronger, although I can see how it might be perceived that way certainly.
Somewhat more common would be the combination of the yuan source with the luo of the paired meridian. But again, there has to be a precise purpose you are trying to achieve for the individual patient.
Whenever you use a point, or set of points, you yourself need to have in mind a very clear idea of why you are using it. Your physical and intellectual intent is a huge part of the overall success of the treatment. Generally speaking, less random stimulation is the most effective so every single point, every touch has to be carefully considered and only done for a specific purpose for that specific patient. Much of this ability builds slowly over decades of practice and study.