here's a growing list of apps that support PiP and splitview.
http://forums.imore.com/ios-9/340612-list-ios-9-apps-support-split-view-ipad.html
1Password: Slide Over / Split View, Spotlight
1Writer - Slide over/split view and Spotlight search
Adobe Comp CC: Slide Over / Split View
Adobe Creative Cloud: Slide Over / Split View
BeWeather 2 (only paid version) - also has Slide over / Split View support so it seems (wasn't mentioned in update)
Booklover: Slide Over / Split View,
Box: Slide Over / Split View, Spotlight,
Calculator for iPad Free: Slide Over / Split View
Calendar Z - Slide over/split view, Spotlight search
Chrome: Slide Over / Split View, Picture in Picture (PiP)
Clips - Slide over/split view
Crossy Road: Slide Over, Split View
Dailymotion: Picture in Picture, Spotlight, Deeplinks
Davka Hebrew Writer: Slide Over / Split View
Digg - also has Slide over / Split View support so it seems (wasn't mentioned in update)
Disney Movies Anywhere - Picture in Picture (PiP)
Evernote: Slide Over / Split View
Feedly: Slide Over / Split View,
File Manager by Mousavian - Slide over/split view and Spotlight search
FileBrowser: Slide Over / Split View, Picture in Picture (PiP) *
FileBrowserLite - PiP only (not yet the paid version probably still in process at Apple)
FileMaker Go 14 - Slide over/split view
Flipp: Slide Over / Split View
HBO NOW: Picture in Picture (PIP), Spotlight
Heapo - Slide over/split view
Instashare Air Drop - Slide Over / Split View,
jetAudio Music Player: Slide Over / Split View
MacID: Slide Over / Split View, Spotlight, App Thinning,
MeisterTask: Slide Over / Split View, Spotlight, (Quick Actions, Peek and Pop)
Messenger (by Facebook) - Slide over/split view
MindMeister: Slide Over / Split View, Spotlight, (Quick Actions, Peek and Pop)
MindNode - Slide over/split view and Spotlight search
Music Launcher: Slide Over / Split View
myMail - Slide over/split view, Spotlight search,
Newsify - Slide over/split view
nPlayer: Picture in Picture (PiP) - Quick Time videos only
Onefootball: Slide Over / Split View, Spotlight
OnePad Pocket Notebook: Slide Over / Split View
Overcast: Slide Over / Split View,
Pingpad: Slide Over / Split View, Spotlight,
Pixelmator - Slide over/split view
Plex - Picture in Picture (PiP)
Pocket - Picture in Picture (PiP)
Pocket Informant: Slide Over / Split View, (Quick Actions, Peek and Pop)
ProTube for YouTube: Picture in Picture (PIP), Slide Over / Split View,
Screenshot++ - Slide over/split view and Spotlight search
Skype for iPad - Slide over/split view
Slack: Slide Over / Split View
SLPRO: Slide Over / Split View
SnipNotes: Slide Over / Split View, Spotlight,
Soundrocket for SoundCloud - Slide over/split view
SwiftKey Keyboard: Slide Over / Split View
Telegram - Slide over/split view, Quick Reply
Trello: Slide Over / Split View, Spotlight,
Tweetbot 4 for Twitter - Slide over/split view, Spotlight search, Quick Reply,
WatchBeam: Slide over / Split View / Picture in Picture (PiP)
WatchESPN: Picture in Picture (PiP)
WhatsApp: (Peek and Pop, Quick Actions), Quick Reply (only works in iOS9.1 - currently in beta)
WWDC: Picture in Picture (PiP)
You Doodle: Slide Over / Split View