RxJS - split Observable into two, wait for first to finish

I have an Observable that looks more or less like this:

-a--b-c---d--e----f--->

where a, b, c, d, e, f are emitted values.

I want to split this Observable into two Observables, in such way that the last emitted value is "alone".

-a--b-c---d--e-------->

------------------f--->

Now, I want to wait for first Observable to completely finish (even inner Observables), after that, execute second Observable.

I know, that I should probably use concat, but I'm actually struggling with splitting Observable into two