Hey,
I think you'll find that the general concensus is that you should make sure you have a suitable home before buying the snakes, but anyway its done now. I don't own any of these guys (wish I did though), but I am quoting a reliable source and I am sure those here with BHPs will sort you out...
According to John Weigel's book 'Care of Australian Reptiles in Captivity' (you should get yourself a copy of this :wink
, BHPs are mostly ground dwelling so you wont need anything with a great amount of height, and the following is a quote from the book:
To house one, or occasionally two specimens, an indoor terrarium should be at least half the length of the longest specimen within; the minimum width should be half the measurement of the cage length.
Considering that a yearling BHP is probably going to be at least 4ft long, you'll want something about 600mmW x 450mmH x 300mmD according to the book, but if it was me I would go 600mmW x 450mmH x 600mmD. This way there is a little room for growth before needing something bigger, and if you are keeping them together you may need something a little bigger again, possibly 900mmW. Again, I am no expert, just going off the book...
Also, the book mentions that large specimens (meaning adults I guess) can be cannibalistic around feeding time, not sure when you'll need to start worrying about this but at least be aware of it...
Cheers.