I have `2 observables` that I want to `combine`, and use their combined version inside a `combineLatest` with other `observables`.

I'm not sure how to `combine` them correctly for my Use Case.

First of all let's call them `ObsA` and `ObsB`, and their combined version `ObsCombined`.

```
this.api.ObsA(), 
this.api.ObsB(), 
this.api.ObsCombined()

```

I need that their combined version only `EMITS` values following these rules:

* `ObsB` is a `BehaviourSubject` and its starting value is `Null`.
* when `ObsA` emits, always make `ObsCombined` emit
* if `ObsA` emits "`null`" as next value, then `ObsB` value must be reset (I will call `observable.next(null)`), and `ObsCombined` should then emit both `null` values

So basically, `ObsA` is the most important, and the special thing is, when it emits `Null`, I want to make the other `Obs` reset and emit `Null` as well.

If `ObsB` emits a value, and `ObsA` last emitted was not `Null`, then emit this new value and the previous one from `ObsA`