Combining Drupal & WordPress on a Multi-Site Project
I would advise reconsidering your proposed solution and making sure any benefits are worth the potential extra effort/hassle, because I don't really see any pros for doing it and there is a very large con of complexity (for no real gain).

In terms of your current reasoning for wanting to do this:

Advantage 1: Using two different CMS's will help me differentiate the sites.

Why not just use different themes for different sites to differentiate them?
You could have different themes for admin, or front end, or both, depending on what you aim to achieve.
The themes could be totally different, or they could just be the same theme in different colours, or something in between.

Also, wouldn't it be easier if the editing/admin experience is the same for all sites instead of being different?
This also goes for any user interaction with the site from non-admins.
Consistency is an excellent thing.

Advantage 2: A special feature not available on one CMS might be available on the other CMS.

Both platforms are very mature and can handle most things. If you want to do something really different you're probably going to have to write code to do it regardless of the platform. If you have two different platforms and you want to do something custom on both that means you have to write both a drupal module and a wordpress plugin (or modify existing modules/plugins for both platforms), so you're making more work for yourself.

It also sounds like the special features you talk of are things you might think of in future.
If you are choosing platforms try to think of all the features you want first (and features that maybe you might want in future), then choose the platform that is best for implementing those features.
Obviously you might think of things in future that you didn't originally think of but there isn't much you can do about that so the platform you consider the most flexible, and the platform you prefer to do custom code for wins points in this regard if you think that complex new functionality might be required in future (remember here that a large amount of commonly used functionality is available on both platforms).

Now there are the disadvantages, which include but are probably not limited to (I don't know exactly what your plan is for these sites to work together or interact with each other but these things are still likely relevant):


Possibly doubling effort when you have to write custom code (as mentioned above).
Adding effort when doing maintenance, like module/plugin updates.
Added complexity, which could make it harder to develop the new features you come up with in future, could make it harder to track down problems when they occur, could make it harder to use the sites (or understand how they work together) in general.
System requirement mismatches. These two platforms require basically the same things but sometimes to do something on a given platform you need a specific version of PHP, or MySQL, jQuery versions etc. Getting two platforms sorted out with specific library versions will be harder than one (if they have to match, maybe they don't).


Basically I think it is best to achieve what you want in the simplest way possible.
Sometimes adding complexity is necessary for the requirements, or you may be able to create something that much better and it will be worth the effort/hassle but if not, and you are adding complexity and gaining nothing of value in return (or the potential value doesn't outweigh the potential troubles), then you are losing out.