You can use AVAudioFile for that. Below is an example. The firstHalfUrl and secondHalfUrl variables are URLs where you'd want to save the first and second halves of the original file.
Instead of saving smaller files, you might want to append(_:) the AVAudioPCMBuffers directly to a SFSpeechAudioBufferRecognitionRequest.
let url = Bundle.main.url(forResource: "file", withExtension: "ext")
let file = try AVAudioFile(forReading: url, commonFormat: .pcmFormatInt16, interleaved: true)
let frames = file.length / 2
let buffer = AVAudioPCMBuffer(pcmFormat: file.processingFormat, frameCapacity: AVAudioFrameCount(frames))
try file.read(into: buffer, frameCount: AVAudioFrameCount(frames))
try AVAudioFile(forWriting: firstHalfUrl, settings: file.fileFormat.settings).write(from: buffer)
file.framePosition = frames
try file.read(into: buffer, frameCount: AVAudioFrameCount(frames))
try AVAudioFile(forWriting: secondHalfUrl, settings: file.fileFormat.settings).write(from: buffer)