The problem with asynchronous planning is that you're missing out on the chance to work harmoniously with your co-teacher. Plus, some things are easier to convey through voice, rather than text. A workaround for this method is to use a voice-based technology tool so your co-teacher can hear your explanation in their own time.
Mote is a chrome web extension. You can record voice comments in Google Docs (in addition to other Google tools!) This tool is great for co-teachers planning because you can leave audio comments to help convey your thoughts to your partner. Just highlight the text, create a comment, then click the Mote logo to start recording.
Voxer is an app. Once the app is on your device, you can create messages to your co-teacher. You can record voice messages that they can listen and respond to within their own timeline. It's great for brainstorming lessons ideas or discussing weekly plans. Instead of going back and forth in texts or emails, you can just use your voice.
Loom is a screencasting tool that is either a Chrome extension or a desktop app. Using this tool, you can record a quick video for your co-teacher. Once the recording ends, a URL to the recording is automatically saved to your clipboard. You can paste it into a Google Doc comment. Your co-teacher can either click the link to view the recording or hover over the link and click Play to watch it within the Doc. The immediacy of the sharing makes it an easy way to brainstorm and communicate.