Never drive your speakers harder than they are rated for - you will destroy them long before a properly sized crossover component. The information given here implies that you can only use a low frequency speaker and a high frequency speaker. This is how most pre-packaged 3 speaker crossovers are made. Exceptions to the rules Piezo Tweeters appear as a capacitive load to your audio power amplifier, and their actual load is a function of frequency. Piezo tweeters typically cannot handle more than 50 to 75 watts, so you normally wire them up in series until the needed power rating is achieved.
Many people consider their sound harsh.
I like them, but I don't use that many of them in my PA cabinets. Piezos can radiate high frequencies at high power so well, that they can damage your hearing without you noticing it. They are also highly directional - You may need a few to aim in different directions to properly disperse the sound.