Get a cable like this one. Male on both ends. Plug it in place of your headphones. Many people have one to connect telephone to car stereos.
You can use the other end to check voltage output with a multimeter. It can be a bit fiddly, but not terribly so. Put the black lead on the sleeve or ground portion. Put the red lead on one of the other parts of the plug to check either right or left channel. If you have balanced headphone outs, then this will be different.
Find a comfortable volume for your test with one of your DACs. Without changing volume send it a test tone, 1 khz is fine, 440 hz might be better for a cheap multimeter. I'd suggest a -6 db tone. Write down the voltage. Switch to your other DAC, and sending it the same test tone, adjust volume on it to match the voltage reading with your first DAC. Get the two within 1% of each other. If you read 1.oo volts then you want something between .99 and 1.01 volts. Takes longer to read these instructions than to do it once you have done it a couple times.
Now as long as you don't change volume on either DAC you can switch back and forth with matched volumes. If you have a headphone amp you can even change volume on it as long as the DACs are putting out the same voltage/volume.
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