Вы можете использовать операцию объединения.
scala> val rdd1 = sc.parallelize(List((1,2), (3,4), (5,6)))
q: org.apache.spark.rdd.RDD[(Int, Int)] = ParallelCollectionRDD[1] at parallelize at <console>:24
scala> val rdd2 = sc.parallelize(List((7, 8)))
q: org.apache.spark.rdd.RDD[(Int, Int)] = ParallelCollectionRDD[1] at parallelize at <console>:24
scala> val unionOfTwo = rdd1.union(rdd2)
res0: org.apache.spark.rdd.RDD[(Int, Int)] = UnionRDD[2] at union at <console>:28