Using rxjs operators, conditionally combine results from two Observables based on a predicate regarding values from the first Observable.

conditional-combine.ts

import { iif, of } from "rxjs";
import { map, switchMap, withLatestFrom } from 'rxjs/operators';

const values1 = [1, 2, 3, 4];
const values2 = [5, 6, 7];

const values1$ = of(values1);
const values2$ = of(values2);

values1$.pipe(
  switchMap((result1) => 
    iif(
      () => result1[0] === 1,
      values1$.pipe(
        withLatestFrom(values2$),
        map(
          ([first, second]: [number[], number[]]) => [...first, ...second]
        )
      ),
      values1$
    )
  )
).subscribe(console.log);

// [ 1, 2, 3, 4, 5, 6, 7 ]