Two service providers with two different service durations.

I have been around the forum looking for an answer but I can't seem to find exactly what I'm looking for so here goes.

I have 2 service providers and each offer the same service and the same duration of appointment starting on the hour.

Now, one provider wishes to change their service to 90 mins which can be booked on the basis of any free time slot which can accomodate 90 minutes for her service.

The other service provider should not be affected by this change as they wish to continue as they are today.

Any help would be appreciated.