I've been doing some reading on stand development, I'm curious if adding more rodinal developer (1:50 instead of 1:100) or add additional agitation/ time to push film? Just wondering how much more developer I would need to push, say, Ilford hp5 120mm +2 stops?
You usually push by keeping the film longer in developer or using a different temperature for the developer. I haven't yet heard of anyone using more concentrated developer for pushing.
You also have to consider what developer you use. Concentrations and times might vary from one developer to another.
It is normal to use an active, “speed-enhancing” developer for push processing, such as ILFORD Microphen or ILFORD Ilfotec DD-X, and also to develop for a longer time or at a higher temperature than normal; see the fact sheets for individual ILFORD films or the ILFORD Film Processing Chart for development time guidelines.
Your choice of developer will influence the quality of your image as much as your choice of film. Most developers can be used for push processing, but a few, such as PERCEPTOL, are unsuitable as they lower film speed in order to obtain the finest grain. The best push processing developers, such as MICROPHEN and ILFOTEC DD–X, give a small increase in film speed at normal contrast.
In any case, you have to check the fact sheets for both film and developer before starting developing the film, no matter you do a normal development or you want to push.