Answer: You can plant a second apple tree now, although it might be stressful, unless you can water it deeply once each week. Or, you can wait until fall to plant your second tree. Choose one that will bloom at the same time as your current apple tree.
Just a thought - sometimes apple trees go into a cycle called alternate bearing in which they produce a huge crop one year, a small crop the next year and then a huge crop the following year. This might be something to consider. Perhaps there are already neighboring apple trees and your tree was on an 'off' year. Think back to last year and try to remember whether or not you had a good harvest. If so, but this year is bad, I suspect your tree has cycled into alternate bearing. Having another tree in the yard won't change this cycle but winter pruning and thinning of the crop in early June can make a difference in the number of fruits your tree can develop to maturity.
Best wishes with your garden.
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