Hi.

I've searched around for a while now, and it seems that such an extension doesn't exist.

Making a mock-up document where I've replicated the wanted outcome is much more easy and probably also easier to understand.

Here is a real-life example:

• I sometimes use Inkscape to manually draw PCB tracks (yes I know there are specialism software out there, but doesn't come with absolute freedom in terms of design).
• When multiple parallel tracks are drawn together, the most efficient workflow for me is to use straight angles only. Later on I can adjust the angles.
• Sometimes, I want the tracks to bend around a hole or some obstacles.
• And yes - I surely can use Corners path effect.
• But - using same diameter on all rounded corners doesn't look very good. So I tend to put time and effort into making the rounded corners having the arc diameter in such a way that the local group of arcs (one corner, multiple paths) are having the same center point.
• This can be tedious and the result are static, i.e. movement of the nodes (i.e. corners must move) then re-adjustment are most likely necessary. 

The attached files

lines and common arc center.svg
Simple sample file (smallest possible description so to not make people bored)

highlighting issues for parrallell lines.svg
This highlights some issues I encounter when trying to create parallell paths with nice round corners. Workarounds are possible but tedious.

steps achieve acceptable parallell lines.svg
This describe my workarounds and it's limitations to gain the wanted result, also why re-editing may cause additional workload to get acceptable result.

Summary

This is a feature request for path effect that make it easier to create parallel paths in such a way that it retains good shape after editing.

If somebody see better workarounds I'll gladly listen to that too.