Getting both curves red is not the problem, but note the rounded edge that I want to have. How can I get this rounded edge? Thanks!
Look for the 'Join" and "Cap" settings in the Fill & Stroke panel > Stroke style. Caps are applied at the ends of curves, and joins at corner nodes.
You may get different results for two shared endpoints (each will have its own round cap) than for a corner (a round join will fill the outer bend).
- Select both lines
- Choose Path/Combine (Ctrl+K)
- Choose the points tool (F2) and select the two endpoints (drag a rectangle around them)
- Merge the endpoints
- Choose Object/Fill and contour (Ctrl+Shift+F)
- Set Join to rounded
This was translated from a German Inkscape. Feel free to change to the correct terms.