Nothing particularly wrong with preblending. Some would suggest premixing the beans and letting them rest together but I store all my beans in the same place so they should be pretty even in moisture content.
You do need to give some consideration to the normal roasting time and level of roast of each bean as mixing big-slow-dark roast beans with small-fast-light roast beans obviously wont deliver the desired result.
I almost always preblend two and very occasionally three beans when Im not doing SO and have had good results. It is a bit of a compromise, but as you note, I dont want stale beans at the end of the batch.
Blending is an art in itself so doing them two at a time in different ratios can be quite a learning experience.