updating certain fields to prevent overwriting

I'm developing a Android-application using StackMob. Now I'm at the point that I want to save an object but not all his properties let's take this example.

class A extends StackMobModel
{
    String UserOneInput;
    String UserTwoInput;
}

Now I have two people using one instance of class A at the same time. User one puts his information in UserOneInput and user two in UserTwoInput. Now if user one saves hits information while user two already has fetched this object the situation would be

class A extends StackMobModel
{
    String UserOneInput = "User one his input";
    String UserTwoInput = null;
}

For user one while user two has

class A extends StackMobModel
{
    String UserOneInput = null;
    String UserTwoInput = null;
}

Now if player two saves his data it's saved as it is in his situation so we get

class A extends StackMobModel
{
    String UserOneInput = null;
    String UserTwoInput = "User two input";
}

User one his input is overwritten. I can fetch the object again before saving but if you use it on mobile networks the latency can still cause the same problem. (User two saves his information between the time that use one does a fetch and save)

I looked into the javadoc and only found a function that you can use to select certain fields but it says that won't work for saving. Is there such an method for saving only certain fields of a class? Or is there some different model I should use to prevent overwriting?

Solution

The class does not know who is the owner, in order to know which one String to save.
This class must be decomposed to

class Answer {
    String answer;
    ....
}

and

class Question {
    Answer userOne;
    Answer userTwo;
    ....
}

Each user has access to and saves his own Answer and the Question knows the Answers of the users.