Usually component sets come with crossovers that are paired with the speaker capabilities. If you are buying just a set of tweeters, you will want to buy a crossover for that tweeter or an inline bass blocker/high pass filter for it. They just go on the positive speaker terminal. Such as these:
http://www.crutchfield.com/S-BXqGXBUP2Cq/cgi-bin/ProdView.asp?g=761&I=127BB300A2 Most car speakers are 4 ohm. Home speakers are 8 ohm.
If you have an existing set of components and you want to add an extra tweeter on each set, you need to be careful if you use the existing crossover. Just as Chryco said "wiring in series will double the resistance to 8 Ohms" and if go in parallel you will drop the resistance to 2 ohms. Dropping to 2 ohms, effectively doubles the power going to those speakers, and if the speaker can't handle it, they will blow really quick. Not to mention, if you have double the power going to your front tweeters only, it will not sound right and be hard to tune to what does.