Should door locks and deadlocks be keyed the same. Is there any advantage or disadvantage (besides having more then one key) in having them keyed different?
The big disadvantage is having to keep two keys with you instead of just one. If you lose your keys (or someone steals them) both keys are compromised so there's not an advantage to having two different keys in that case.
The only advantage I can see is if you frequently need to let other people into your house. You could hide the door knob key or give it to the third party, and on the day when they need access leave the deadbolt unlocked. That way the door is still locked and the third party only has access when you allow them to. Any other time the deadbolt will be locked so even if someone finds the hidden key they can't use it.
It's a bit contrived, but using the two keys as a "double lockout" type scenario is the only advantage I can come up with.
As with most security questions, this one can be answered with an XKCD comic. Essentially, you're making it complicated for someone who is playing by the rules (EG: Someone who will use keys to unlock the door, or try and manually pick the locks). The people who will break the rules will probably go the easier route of breaking your windows, or using some other easier point of access (Like hitting you with a wrench and taking your keys). A system (house) is only as secure as it's weakest point of entry, and taking time to secure an already reasonably secure point of entry does little to improve your security position.
Note: There may be other less security related advantages, but security is not an advantage of differently keyed locks.
In my former home, I had the standard knob lock and deadbolt keyed differently. When we had our pest control service visit, I left the deadbolt unlocked and put the key for the knob lock in a combination lock box hanging from the front door knob. The service tech accessed the knob key from the lock box, let himself in to do his thing and then left the key on the dining room table and locked the door behind him. While I trusted the service tech, even good people can go bad and if he decided to make a copy of my key one day, it wouldn't do him any good because I always use the deadbolt. In fact, outside of this service day, I kept the knob lock unlocked and only used the deadbolt. The dual key setup worked for me.