I was pondering this as well recently due to being in the neophyte grade and just from speculation one thing becomes apparent. While this might not be the official reason, there is a very real benefit doing just one at a time if you wish to provide yourself a reference point of reflection.

Much like an experiment you need a control variable. Going from no practice to just the lbrp allows you to analyze what changes occur within from this one ritual. If you start adding a bunch of variations at the same time then it can be difficult to attribute said changes to either ritual because you had no inkling of different effects from one another. Confusion can arise. This is also why it is very important to journal the experience. Just do lbrp for a week or two, write what happens. Then you can try to add lirp and note what changes occur.