The easiest way would be to tape the Horizon regular boxes together. It would take 2 different sets of 3 boxes taped together. 6 adults in 1 set (2 per box). In the other set 2 adults in 1 of the boxes and divide up the 1-2 month olds between the other 2 boxes.
Until you pay the cargo shipping on those express boxes, its cheaper in the long run just to buy some of the 2 bird boxes. I can get them as low as $7/box and sometimes even free shipping if I buy enough of them. Even if you figure $10/box, it would take you 6 boxes for a total of $60 just for them. Those Omni boxes are about $25 each roughly and they charge through the nose to ship them too.
When you estimate the express mail, go to www.usps.com and plug in the zip codes and box weight/sizes. It will be a little different for each location. When taping those boxes together, the measurements will run about 16" tall x 10" long x 24" wide (8" for each box). Generally it runs right about $90-95 for those.....
If shipping that many birds, without having an accurate weight or the zip codes, I can give you a rough quote.... It will be about $180 for the express mail and $60 for the boxes. About $240 total. Its ALOT of birds!!! When you figure 1 pair is generally about $60-65, it still better to send more if you can.
You can actually tape up to 4 of the single boxes together. I wouldn't put more than 2 adult silkies in one box, but you could put all the 1-month-olds in one and the 2-month-olds in another. For safety reasons, I wouldn't put the little guys in with the adults.
The omni nest boxes do have 8 compartments but they are designed for pigeons and wouldn't be big enough for an adult bantam. Though if the birds are all going to the same place, you could use one of the swan nest boxes for all of them. They can be divided into 1, 2, 3, or 4 compartments. I haven't been able to find any place that sells just one of those though, except for http://poultry2.tripod.com/id3.html if they aren't sold out. But honestly, it's probably still cheaper on your buyer for you to use 3-4 of the singles taped together.