Can a dual band router send 2.4Ghz/ 5Ghz at the same time?

Just because your set the router up to broadcast on 5ghz does not mean your laptop or other devices are compatible. As an example my laptop is 2.5 ghz. To have it SEE the 5ghz router I have to use a separate thumb drive device with the proper receiver.

And yes, they can simulcast both signals.