Second consecutive Session fails to start

Trying to start second session after saving first one. It works on simulator, but doesn't on Fenix3. The code is next:
var s = Rec.createSession({ :name => "Swim", :sport => Rec.SPORT_SWIMMING });
Sys.println("Session: " + s);
Sys.println("Start: " + s.start());
Sys.println("Stop: " + s.stop());
Sys.println("Save: " + s.save());

s = Rec.createSession({ :name => "Swim2", :sport => Rec.SPORT_SWIMMING });
Sys.println("Session: " + s);
Sys.println("Start: " + s.start());
Sys.println("Stop: " + s.stop());
Sys.println("Save: " + s.save());


Output on watch is next:
Session: Obj: 100
Start: true
Stop: true
Save: true
Session: Obj: 168
Start: false
Stop: false
Save: true


However, if I start one more session after some time it will successfully start. I assume it happens because of save() delay on watch. All native app have progress during save/dismiss, triathlon app has stub views between sessions, but I do not see the way how to determine if session is still saving. Please help.