According to Chocolate, it is not additive. The 95% decrease will apply to your Dragon first, and then the Rider heal time will be reduced based on that decreased number.
For example: Let’s say it takes 100 minutes for your Dragon to heal. The Seasons Buff will bring this down to 5 minutes (-95%). Then, Kevana’s heal time reduction will be factored at 15% of those 5 minutes (0.15 x 5), bringing it down to 4 minutes and 15 seconds.