It is not necessary, but it is always a good idea in case one fails. That said, I dont run two heaters in my tanks. A 250w should be fine, I run a 200w in my 40 and 300w in my 150 and 125.
The inline heaters are nice, I really wish I had went with one. I have been really happy with my Aqueon Pro heaters from Petsmart... and they're not very pricey.