Introducing twiin: Stop Double-Booking Across Microsoft 365 Accounts

The Double-Booking Problem

Tuesday, 2 PM. You just accepted a sprint planning meeting on Client A's Microsoft 365 calendar. An hour later, Client B schedules an urgent call at the same time. You accept it too, because you were looking at the wrong calendar.

Here's the thing: when Client B checks your availability in their tenant, they only see your Client B calendar. Your Client A meetings? Invisible. So they schedule right over them, and you end up double-booked.

Freelancers get separate M365 accounts from each client. Contractors bounce between organizations. Remote workers juggle multiple corporate accounts. Three accounts, three calendars, zero visibility between them.

The usual workarounds:

• Just live with it. Accept the occasional double-booking as part of the job.
• Forward every invite to your other accounts. Sometimes you end up telling clients about it, which isn't exactly professional.
• Use a cloud sync tool. Your credentials and calendar data sit on someone else's servers, you need to set up app registrations, and some companies flat-out prohibit these tools.

None of these are great. We wanted something better.

So We Built...

twiin is a desktop app that syncs your Microsoft 365 calendars. It runs in the background and keeps your accounts in sync automatically. You always know what's actually on your plate.

How It Works

Connect your Outlook accounts, and twiin monitors them for changes. Schedule a meeting on one account, and it blocks that time on your other accounts. Works both ways.

You set the sync schedule. After that, it just runs.

Three Ways to Sync

Different situations need different levels of detail:

Free/Busy Mode: Maximum privacy. The other calendar just shows "Busy" with no event details. Good when you don't want specifics leaking across tenants.

Basic Event Mode: Shows the event title plus the time block. Enough context to know what's going on without exposing the full meeting details.

Full Event Mode: Everything syncs: title, description, location, attendees. Use this when you own all the accounts and just want full visibility everywhere.

One-Way or Two-Way

You can sync in both directions (change one calendar, the other updates) or just one-way (primary pushes to secondary, but not the other way around). Your call.

Your Data Stays on Your Machine

This is the big one: your calendar data stays on your machine. Our servers only coordinate syncs using random IDs. No meeting details, no attendee lists, no event content ever leaves your computer.

Your local twiin client connects directly to Microsoft 365 using OAuth. All calendar data is processed on your machine. Our servers only receive anonymous IDs to coordinate what needs syncing. Credentials are encrypted and stored locally.

What this means in practice:

• Your meetings stay private. Our servers only see random IDs to tell your local twiin what needs syncing. Actual meeting content never leaves your machine.
• No admin required. No app registrations, no IT approval. If you can use Outlook Web Access, you can use twiin.
• We can't see your data. Literally. We don't have access to your calendars or meeting patterns.

This makes twiin GDPR-compliant by design. If you're handling confidential client work or have strict data policies to follow, this matters.

Who's Using It

Consultants with multiple clients. Three enterprise accounts, each from a different client. Sync them all together in Free/Busy mode. See your full availability without leaking client details across tenants.

Remote workers. Multiple corporate accounts from different employers or projects. IT won't let you share calendars externally? twiin runs locally, so there's nothing for IT to block.

Agency owners. Your agency calendar plus client-specific accounts. Sync in Basic Event mode so you see "Client A: Sprint Planning" without exposing the full agenda.

Getting Started

Setup takes a few minutes:

1. Download and install the app
2. Connect your Microsoft 365 accounts
3. Pick which calendars sync with which, and choose your sync mode
4. Set your sync schedule
5. Done

After that, twiin lives in your system tray and does its thing in the background.