James - I followed your thread on the Bilgy tent and congratulate you on a beautifully executed job. However, I feel that for me, the level of accuracy in cutting and sewing would be beyond my competence, and I might well end up with something which might (emphasise "might") work, but would look like a bag of laundry. I am also somewhat concerned that the tapers would be bias cut an therefore weaker than the straight cut edge of the top piece. A second consideration was the OP's requirement for room for three. Prototyping the inner will be easier than re-sizing the BILGY, and then a suitably large straight tarp used for the outer - he doesn't have to put on beaks or doors like Jardine, although they would be desirable.
The disadvantage would be the additional weight of the top pieces of the inner tent and also of not tapering the tarp roof.