Hedging with these two shrubs should really be planted early spring, or in autumn, so if you want to do it now, you need to make sure it is kept well watered until late autumn. If you want to mix these two (I wouldn't) make sure the Pyracantha you choose is not one of the bigger varieties, which can reach 15 feet - choose one with a maximum height of 10 feet - Berberis darwinii will reach a height of 6-7 feet max. There is a pyracantha that only reaches 6 feet - P. 'Harlequin'.The other slight difficulty is that pyracantha grows somewhat differently to B. darwinii - the latter is taller than it is wide after five years, whereas pyracantha tends to be wider than its tall at five years. Recommended spacing for pyracantha hedging is 2 feet apart, for B. darwinii, it's 20 inches. Pyracantha should have its lateral shoots pruned back a little at planting (for hedging) and Berberis needs to be reduced by half in height, assuming the plants are over a foot tall to start with.