proteins - What are the most important differences between HSP70 and HSP90?

Often cells have multiple types of the same protein — this redundancy can have different effects for different requirements such as having proteins function under different physiological conditions, or providing specificity to a certain class of ligand proteins or so on.

But here, it seems like the two have some synergistic interaction, a tag team if you will.

Wegele H, Müller L, Buchner J. 2004. Hsp70 and Hsp90 — a relay team for protein folding. Reviews of physiology, biochemistry and pharmacology 151: 1–44.

Unfortunately this article's full version can only be accessed if you're at a university or somewhere that has a subscription to some of the large research databases, but the abstract is free and it may provide more clarification.