Some very common causes of camellia bud drop are weather related. Wild temperature swings and lack of constant or uniform soil moisture can trigger these. One shrub can withstand them better than others so proximity may not be a problem but too much shade might make the plant more sensitive to problems like temperature wings, periods of dry-wet soil, soil that does not drain well, or even problems with the camellia bud mite (spray with a miticide if there is evidence of pests), etc.

So basically, when a plant is stressed by one those factors, you can have bud drop. Since your problem occurs regularly, evaluate which one (ones?) of these factors is contributing to the problem and take appropriate action.

I would not place the camellia too close to the other one. Maybe keep them 3-4 feet away.... depending on whether you are trying to build a hedge and things like that.