Solution:

ExecutorService executor = Executors.newFixedThreadPool(3);
CompletableFuture.allOf(
        CompletableFuture.runAsync(() -> first.actions.stream()
                .sorted(Comparator.comparingInt(a -> a.priorityId))
//                        .peek(a -> System.out.println("START: " + a.priorityId))
                .forEachOrdered(a -> processAction(first.entityId, a.priorityId, a.actionName)), executor),
        CompletableFuture.runAsync(() -> second.actions.stream()
                .sorted(Comparator.comparingInt(o -> o.priorityId))
//                        .peek(a -> System.out.println("START: " + a.priorityId))
                .forEachOrdered(a -> processAction(second.entityId, a.priorityId, a.actionName)), executor)
);
executor.shutdown();