Hi MIchiel,
As an aside, as a player myself, I find it easier on the eyes to read the out of phase version - it is perceived as two separate entities rather than a single double line, if you see what I mean.
As to how to do it, if you used \draw-dashed-line in a markup you can specify the dash pattern and the phase. I can't recall offhand if this applies to text spanner lines as well, but it may do.
Andrew