The total impedance will drop when running 2 pair of speakers. Do you know if your amp has an over-current protection circuit? If there is no over-current protection I would NOT do it. The Dyns are 4ohm. If the Focals are 4ohm that would be a 2 ohm load to the amp. Years ago I had a Denon pma700 that I would drive 6ohm and 8ohm speakers together and at loud listening levels the Denon would shut down. Normal levels were fine.
Just my experience.
Just my experience.