I agree with Wes's reasons for two and the need for it to be on a controller of some sort.
Titanium heaters have taken a shocking downturn in quality the last year or so. When they first came out, they were bullet proof, now its a crap shoot. I've seen many a post from people who had one go bad on them or zap their tank, leak copper etc. Jason at Premium Aquatics steered me away from them due to the issues they had with Won and Pro-Heat. He suggested the Hydor Theo because they just don't break and are very reliable.
http://www.marinedepot.com/ps_AquariumPage~PageAlias~heaters_hydor_theo.html
You can get two of the 150W and plug them into one of these... Turn the temp up on the heater to about 2 degrees above where it should be, then set the controller. It will continue to heat and the controller will shut them off when it reaches temp and if the controller fails it will only go 2 degrees above. Only time you would get into problems is if both controller AND heater failed.
http://www.marinedepot.com/ps_ViewItem~idProduct~AM1713.html
http://www.marinedepot.com/ps_ViewItem~idProduct~AL1713.html
http://www.premiumaquatics.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=TW-DC&Category_Code=GroundTemp