Alun_evans is right on target about moisture content. Here's what I would add...before you roast different beans in the same batch, blend and store them for 24 to 48 hrs - this will help the moisture to balance. Put them in a Tupperware, shake them around and leave them.
I'm like Alun - I usually blend post-roast, especially if the beans are different sizes and hardness. Roasting them separately gives me a more even roast for each type of bean.
Fluid-bed (hot air) roasters may have better success roasting blended green coffee.
AS Alun_evans said, pre- or post- is probably less of an issue for a home roaster than a larger commerical batch roaster. The only time I blend pre-roast is if it is a small batch of our house blend and the customer is waiting while I roast. I keep a few pounds blended "in the green" for that purpose.
Happy roasting!
-Steve