[pcho614]

I'm looking to build my own pair (mod) with my ath pro 700s. In no way am I an audiophile, while the pros give great bass, I'm looking to add smaller drivers to help with the highs and was wondering if this type of setup is possible

 

[NA Blur]

Yes this is possible, but the implementation becomes problematic due to crossover frequencies, phase, and how each driver is controlled ( i.e. separate controllers ).

 

[Punnisher]

There are some headphones that utilize this design. If you are using two full-range drivers, you need to keep them equidistant from the ear.
 
The wavelength of a 20kHz signal is about .67" long. If the drivers' distance from the ear is different by half of this, you will experience cancellation in the upper treble. You would need to keep the drivers as close as possible to equidistant for this to work.
 
If you are using a crossover with a two-way design, it becomes less of an issue. Each driver is producing a different frequency so there is less of a concern for cancellation. In addition, the crossover point would likely be in the mids somewhere, which has a longer wavelength and therefore more forgiving.
 
In the event that your tweeter is more sensitive than your woofer, you can use a small L-pad circuit or potentiometer to bring it down to the same level as the woofer.