I found this description of specifically Faroese preaspiration:
http://www.lingfil.uu.se/personal/petur/pdf-filer/0868.pdf
I don't think it answers your question about postaspiration on preaspirated consonants, but I read through it quickly, so I may have missed it. What it certainly does show is the very interesting fact that some Faroese people use preaspiration on ungeminated medial fortis plosives following non-close vowels, e.g.
bátur "boat" = [pɔaʰtʊɹ]. However, there may be both individual and regional variation involved.
There's been a ton of research on postaspiration in North Germanic languages. I hope this doesn't come off as being off topic, but it feels necessary to point out that the dialect of Gräsö in Sweden has been demonstrated to have precisely what you're talking about: preaspiration and postaspiration on the same segments:
http://www.google.com/url?sa=t&sour...E_yUEBWATCP1AgPhA&sig2=EiYQ5oZKpq_zfqTTbeIc8A