I2S DAC hat AND USB dac together

I have been using a Pi3B for over two years now with an IQaudio Digiamp+ for my speaker output and have always been pleased with it and had no major problems. I have been thinking that I really would like to have a headphones output as well. Is there a way of running a USB DAC as well as the DAC hat so that I could choose between the two outputs?
I have got the volume level controlled by a rotary encoder which I am thinking would control volume on a USB DAC in the same way as it does for the DAC/AMP. hat.
I would be grateful for help on this