The standard is - are the claims “patently distinct” from each other. This is like a 103 obviousness analysis (complicated, arguable and somewhat subjective) and therefore there would not likely be a hard and fast rule one way or the other about elements in separate dependent claims. If there was a rule it might go against your position. If the widget can be usefully combined with X and the widget can also be usefully combined with Y it might be obvious that it can be usefully used with X and Y at the same time. And, if the widget can functionally have X and Y together added to it then it might be obvious that either could individually be added to it. You are free to argue that something about the nature of the widget, X and Y make using them separately and using them together are not obvious from each other. If the claims were more alike you might get a statutory double patenting rejection that would be worse.