The bass note is naturally going to sound a little louder because it has more mass & it's an open string (more natural harmonics) constantly ringing. Another issue common to beginners is not fretting the notes on the upper string solidly enough to make them ring clearly enough to be heard.
There are 2 ways to counteract this & balance the volume of the notes on both strings:
Use much lighter picking technique, just graze over the top of the strings, do not dig in to the plane of the string so far.
Lightly palm mute with your picking hand palm on at least the lower string of the diad riff.
I cover playing this riff in this tutorial.
https://guitartricks.com/tutorial/189
Hope that helps!
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