Dual band routers

Hi, guys! Could anyone please clarify if modern dual band routers, which are described to work simultaneously on 2.4 ghz and 5 ghz networks, would have maximum advertised speed on each band when there are clients on both bands? (I understand that max speed is not achievable due to many other reasons)

What I am concerned about is that routers like Asus AC-88U have 4 antennas. These antennas are dual band. I thought that 1 antenna is required for each spatial stream. If 5 ghz network is...

Dual band routers