Topband: Make 80m vertical work on 160

[Franki ON5ZO]

Gentlemen, please your advice.

If I were to convert my working resonant 80m vertical into a 80/160m 
antenna, I assume I have two basic options according to the literature:

- Use a trap to 'split' the antenna on 80m (resonant at geometric mean of 
160/80m frequencies), or
- Use a loading coil that can be switched in/out of the vertical to lengthen 
it electrically.

I was leaning towards the coil for simplicity of construction, yet it needs 
to be at the feedpoint for practical reasons, as it seeds a switching 
circuit with control cable.

Does the trap have advantages over the coil, apart from not needing a relay? 
Where do I start for practical dimensions to obtain a calculated L value for 
the loading coil?

Then I stumbled across the linear loading solution in ON4UN's book. I could 
go up and down once with the verttical wire before going up all the way, and 
switch the loading part in or out for 160/80.
But I didn't really see a practical approach for the wire spacing. How far 
would the loading wires need to be away from each other and the main 
vertical wire. Would 30cm (12") be enough?

Your practical input would be appreciated.
73 de Franki ON5ZO