Double wide double fibre vs Two strips
Below is a simple comparison: imagine two MINOS-like strips, 4cm wide.
Now imagine a single double-wide (8cm) strip with two fibres in it.
How well do these two cases compare?
First, here's the MINOS strip for comparison. These are photons surviving to the fibre as a function of the true start position of the photon. These are each 100k photon trials.
Now, here's my double-wide strip. I've put the two fibres in the of the strip, at +/- 3.8 cm from the middle.
Performance:
The absolute performance of the single strip is 18.9%.
The absolute performance of the doublewide strip is 17.8% (2 x 8.9%)
Discussion:
This indicates that this design actually loses very little light. In fact, you no longer have the few-mm gap between the two strips, which may actually increase your absolute efficiency.
However, this is an attendant loss of tracking resolution. This could be mitigated by doing a light-sharing analysis of the two fibres: i.e.
y ~ (fibre1 - fibre2)/(fibre1+fibre2)
How well would this work? Depends on poisson statistics. In a MINOS strip, you get about 3.5 p.e. of light at 4m per strip end (for a normal-going MIP). That means that in this case, you would get only about 1.7 pe per fibre in the double-wide case. Not really enough to provide a good tracking resolution; you're better off with the single strips.