they both started as an offshoot of homeopathy. There were two practitioners who thought it was silly to wade through all those pesky remedies and wanted to make things easier, lol. (Not true of Bach-I'm just being silly.) The biochemic system is more material, the flowers are more spiritual. But homeopathy is the REAL medicine. LOL.
Seriously though, Bach wanted to put healing in the hands of the masses. He wanted a system that was elegant, safe and effective. He didn't believe people had to be reliant on a practitioner in order to find wellness. Shuessler had a different endgame, he DID want to simplify, but regardless what he did was create a system that anyone could use and understand.
If you take them both you have the physical aspect and the spiritual/emotional and there IS crossover. Cell salts have mental indications too!
There are lots of times when cell salts and flowers won't really cut it and you'll want homeopathy. That's for sure. I think it's a serious mistake to undervalue the salts and flowers as a result though.