I think the signal going to the tweets is full range from the Bose amp (assuming you've got the Bose system). That means there needs to be a cap (or some sort of bass blocker) on the tweeter to only pass the high freqs. Some aftermarket tweeters will have them built in (especially some of these cheap add-on tweeter kits), so you may be ok. If it is a high-end set from a component set, you'd probably have to add something.
A Pair of coax speakers that have built in tweeters plus a second tweeters usually ends up being way too much treble, along with a lot of high end cancellation. This is one big reason why components are a much better way to go.
However, if you decide the only way for you to go is to get aftermarket tweeters and add them on, most tweeter sets will come packaged with some type of high-pass filter to use. Do make sure before you hook them up though. If you hook a tweeter straight to a full range signal, it's life will be spectacularly short.
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