They are technically interchangeable, but I use them differently.
I use accordingly like it's the effect of a cause. Like "When we get new information, we'll react accordingly." Or "If this department decides they'll do their own reporting then we'll respond accordingly." As in tangible action.
I use therefore sort of like "so", but I don't use this word a lot. It is a little formal for my taste and I try to be as casual as possible, even in business, to make language accessible. In most instances I can think of you can omit this word and not lose your meaning.