I used finasteride for nine months, then used dutasteride for 6½ years, and now I'm switching back to finasteride. I've never used them both simultaneously. Indeed, there is no point doing so, because dutasteride will just cancel out the effect of finasteride as it is the stronger of the two. Dutasteride does everything finasteride does and more.
In my case, the reason I've finally decided to switch back to finasteride is because in hindsight, there was no need to change to dutasteride in the first place, as finasteride was working for me and I didn't notice any additional gains after making the switch. Also, I've experienced an increase in fluid retention (stomach bloating and facial puffiness) upon using dutasteride. I had this side-effect to a small extent on finasteride, and it got significantly worse on dutasteride. It is definitely the medication that is the cause because its onset and the increase when switching to dutasteride was very sudden and was clear to observe. The cause is most likely the spike in estrogen, which is why I am about to start taking the supplement DIM, which is supposed to help reduce excess estrogen.
The stories you hear of dutasteride "ruining the temples" are anecdotal reports that are not rooted in science. Seeing as dutasteride gets rid of the primary hormone responsible for hair loss, it is illogical to believe that dutasteride can actually have a negative effect on your hair. Removing DHT (by using dutasteride) can only possibly have a positive effect on your hair. Some people may argue that the spike in testosterone caused by using dutasteride can damage the hair. However, the potential negative effect on the hair of the increase in testosterone from using dutasteride is greatly overpowered by the positive effect on the hair of its extremely potent suppression of DHT. DHT is many times more destructive to the hair follicles than testosterone. Another argument some people may put forward to support the claim that dutasteride (or finasteride) can have a negative effect on the hair is the theory of reflex hyperandrogenicity. However, Kevin Mann does a pretty good job of debunking that theory in this video: