It is quite common for the first two vehicles on a minor arm to queue side by side, especially if the turning proportions are fairly well balanced.
In such cases the PICADY model would normally be set up with a short flare, length = 1 vehicle. This is as it should be, but the result may be unexpected. It might be expected that even a short double-queue would have a greater capacity than if vehicles were queueing in a single file, but PICADY sometimes predicts otherwise.
Visibility is the problem. Each vehicle at the give-way line is obstructing the visibility of the driver alongside, making people slower and more reluctant to emerge from the side road. PICADY accounts for this loss of visibility, which does indeed occur in “real life”. So don’t be too surprised if your extra little bit of flaring at the give-way line makes the junction less efficient ! If you can find room for, say, three or more vehicles side-by-side it is a different matter entirely.