Hi there!
They can be used together, but it's perhaps overkill. Cessorex comes in granules, which are more potent, but Happy Wanderer is generally the first pick. Cessorex is more balanced in its effect, which means it is not as strong a Liver Blood tonic as Happy Wanderer, whose anti-inflammatory and decongesting effects are milder. BUT Happy Wanderer is not as strong as the granule version called Xiao Yao San (from www.atimetohealherbs.com). If I had both Cessorex and Xiao Yao San on my shelf, I'd try XYS first. If the owner finds it easy to give Happy Wanderer even at the inflated dose of two or three times the label dose, I suppose she could start there.
Hopefully, I didn't muddy the waters for you!
Steve