Do I need to get another visa waiver?
No, you don't. On the other hand, the border officer can grant you a fresh visa waiver when you re-enter the US. From your point of view, however, it makes no difference.
All you really need to worry about is having valid ESTA authorization. If your ESTA application is approved, the resulting authorization is valid for two years, or until your passport expires, whichever is shorter. It can be used for an unlimited number of entries while it is valid.
Historically, the visa waiver program (VWP) has been around far longer than ESTA. People often confuse the two, since ESTA is far more visible to travelers than the VWP. It may not be obvious to you, therefore, that you actually receive your visa waiver from the immigration officer who stamps your passport. Whether the US considers the second entry a new waiver or a continuation of the first one is not significant to you.
The VWP does have rules about traveling to Canada that can be of concern to some visitors. These rules are designed to prevent people from using Canada to extend their VWP stay in the US beyond the 90-day limit. Since your entire trip in North America doesn't even come close to that limit, you do not need to be concerned about these rules.
There is of course a small chance that a border officer would question you about your plans to determine whether you are trying to abuse those rules. It would therefore be a good idea to be able to show your entire itinerary when you return to the US, in case the officer presses the point. But the usual principle applies: don't volunteer anything; don't hide anything.