Warsaw is 1 hour behind Moscow
Best overlap: 9:00 AM to 6:00 PM in Moscow is 8:00 AM to 5:00 PM in Warsaw.
Book within 9:00 AM to 6:00 PM Moscow time (8:00 AM to 5:00 PM in Warsaw) and both teams will be at work. That's a roomy 9 hours of common time each day, so finding a slot that suits everyone is rarely a problem. For part of the year the maths changes: Warsaw observes daylight saving, Moscow does not, shifting the overlap by an hour.