I don’t think you’ll be able to do this with a text style expression, as style is applied to the whole text layer, not a partial string.
If you want one name regular (or light) and the other bold, I think you will need to make them separate text layers and then use a sourceRectAtTime() expression to make the second text layer follow the first, as if it were a single text layer.
Create two text layers and parent layer 2 to layer 1.
Add this expression to the position property of of layer 2:
rect = parent.sourceRectAtTime() ; x = rect.left + rect.width ; [ x, value ]
Layer 2 should now follow layer 1 as you modify layer 1’s source text as if they were both on the same text line.
For my set up, i was thinking you’d want each text layer lined up by their y-axis (like it is on a single line of text), and then the expression would go on the Position property rather than the Source Text property.