I would just usually adjust my transfusion schedule, when I was on 4 weekly transfusions I would have either added an extra transfusion in just before I go away or take extra units of blood. I now have blood every 2-3 weeks and get 2 or 3 bags, and at such times as Christmas or holidays I would either go a week early or a the month before a week later for blood. I know that one unit of blood will take me approximately a point higher (eg. from 9- 10 hb) and 1 unit will last around a week (or 4 units will last 4 weeks). I guess everyones different but this formula works for me and allows me to make holiday plans without worrying about needing blood whilst away.
Joe, I think you basically needs to look at how much blood is taken now and how long it lasts and simply add extra blood. It sounds like doubling the blood will make you last around 4 weeks. Just make sure the transfusion is within a few days of leaving and get a hb check within a few days after arriving home.
I personally have never gone longer than 4 or 5 weeks in between blood transfusions so I'm not sure if 40 days is possible. The only thing I think you have as an advantage is that you seem to only need 400ml of blood to last 2 weeks, therefore it might be possible to take 3 or 4 bags before you leave. I would definately refer you back to your doctor for advice because I really can't see how you would manage to last 40 days without blood, I certainly couldn't last longer than 5 or 6 weeks without my hb dropping into the 6's and lower. This also wouldn't make your travels very enjoyable or safe. Sorry I couldn't help anymore x