If I make 1.25 gallons of mead with Lalvin 71B, and put half in one gallon carboy for secondary and half in another gallon carboy. To fill the carboy I will use fruit juice or fresh fruit, possibly mixed with some honey. I know that by giving the yeast more sugar, they will proceed to ferment more, but will it form a considerable krausen? Also, are there any problems in doing this that I may be overlooking? I want to do this to experiment with different flavors. I will be measuring the amounts of ingredients put in each of the different carboys to record.
It is hard to predict the krausen size, as it depends on many factors such as the temperature/speed of fermentation, number of yeast cells pitched.
Depending on the fruit, it can create a big krausen, read this: Fruit mead - VERY vigorous fermentation
Make sure to clean the fruit skin to avoid contamination. Mixing fruit with mead will work, you can ferment separately or together, but if you do it separately you will be able to define a different ratio that you can test/taste before blending.
If you ferment in a carboy, with its shape it accentuates the krausen, it is better to use a bucket as primary fermentor. Transfer to a carboy after the initial krausen if you like.