Pros:
1. Cheaper
Cons:
1. You will only have one set of volume controls.
2. There will be only one input, so both sets of speakers will be playing the same material.
3. The YSMs are 6 ohms. If you wire the speakers in parallel, this will make a 3-ohm load, which you should NOT do, because the amp is rated for 4 ohms.
4. Continuing with #3, wiring the speakers in series will create a 12-ohm load for the amp, which will lower the total power output of the amp. You won't be sharing ~70W (at 6 ohms), you'll be sharing less than that
5. You may require longer speaker cable runs.