Is it possible to make one LiveData of two LiveDatas?

I assume you want to combine them, yes? You'll need a MediatorLiveData, but the guy saying you now need Object is wrong. What you need is a MediatorLiveData<Pair<List<Expense>, List<Income>>>.

public class CombinedLiveData extends MediatorLiveData<Pair<List<Expense>, List<Income>>> {
    private List<Expense> expenses = Collections.emptyList();
    private List<Income> incomes = Collections.emptyList();

    public CombinedLiveData(LiveData<List<Expense>> ld1, LiveData<List<Income>> ld2) {
        setValue(Pair.create(expenses, incomes));

        addSource(ld1, expenses -> { 
             if(expenses != null) {
                this.expenses = expenses;
             } 
             setValue(Pair.create(expenses, incomes)); 
        });

        addSource(ld2, incomes -> { 
            if(incomes != null) {
                this.incomes = incomes;
            } 
            setValue(Pair.create(expenses, incomes));
        });
    }
}

You could potentially make this generic and it'd be the implementation of combineLatest for two LiveData using tuples of 2-arity (Pair).

EDIT: like this:

public class CombinedLiveData2<A, B> extends MediatorLiveData<Pair<A, B>> {
    private A a;
    private B b;

    public CombinedLiveData2(LiveData<A> ld1, LiveData<B> ld2) {
        setValue(Pair.create(a, b));

        addSource(ld1, a -> { 
             if(a != null) {
                this.a = a;
             } 
             setValue(Pair.create(a, b)); 
        });

        addSource(ld2, b -> { 
            if(b != null) {
                this.b = b;
            } 
            setValue(Pair.create(a, b));
        });
    }
}

Beware that I lost the ability to set Collections.emptyList() as initial values of A and B with this scenario, and you WILL need to check for nulls when you access the data inside the pair.

EDIT: You can use the library https://github.com/Zhuinden/livedata-combinetuple-kt (Kotlin) or https://github.com/Zhuinden/livedata-combineutil-java/ (Java) which does the same thing.

Created this extension function in kotlin

fun <A, B> LiveData<A>.zipWith(stream: LiveData<B>): LiveData<Pair<A, B>> {
 val result = MediatorLiveData<Pair<A, B>>()
  result.addSource(this) { a ->
    if (a != null && stream.value != null) {
        result.value = Pair(a, stream.value!!)
    }
  }
 result.addSource(stream) { b ->
    if (b != null && this.value != null) {
        result.value = Pair(this.value!!, b)
    }
 }
 return result
}