I would set up a proxy via using a Subject:
var areAllAvailable = new BehaviorSubject<bool>(true);
PauseCommand = new ReactiveCommand(areAllAvailable);
PlayCommand = new ReactiveCommand(areAllAvailable);
Observable.CombineLatest(PauseCommand.IsExecuting, PlayCommand.IsExecuting,
(pa,pl) => !(pa || pl))
.Subscribe(allAreAvailable);