Emit from single when flowable emits

I have an activity where you can see a bunch of places according to a filter and the current viewport of the map.

So I've modeled these two sources as Flowables:

fun boundariesFlowable(): Flowable<Pair<LatLon, LatLon>>
fun filtersFlowable(): Flowable<FilterRequest>

Where boundaries emits the TopLeft and BottomRight corners of the map viewport, and filters just emits any change in the filters applied by the user.

Now I want to observe these two sources so that a call is performed each time either of them changes. This is my request call:

 fun refreshApplyFilter(id: Int, filter: FilterRequest): Single<FilterResponse>

The problem, I cannot figure out how to combine this trigger sources, or how to perform the call on their emission.

I've tried flatmapping just one of the sources but it expects a Publisher, while my request is a Single.