When/how can one be funded by multiple grants?
What I have seen the most is that the you can have two related projects funded by different grants. They are complementary, but virtually the same techniques are used for both. Officially some people would say that they are different projects and require different funding. Furthermore, you could have different students working on the different projects. In reality, the research aspect of the project (not the results) is the same. For example, you students working in different projects would need the same background and reading material. 

A made-up example can be machine learning. One project can be "solving traffic with machine learning", the other could be "predicting traffic with machine learning". The problems/results/solutions can be presented differently, but the core of the research could be the same.