I noticed and found only first three cases:
We can write $1$ as difference of two composites that have one prime factor $$3^2-2^3=1$$
and as difference of two composites that have two prime factors $$3\cdot 5 - 7\cdot 2 = 1$$
and as difference of two composites that have three prime factors $$2^2 \cdot 3^2 \cdot 43-7 \cdot 13 \cdot 17=1$$
I believe that this holds for every $k \in \mathbb N$, that is, that for every $k \in \mathbb N$ there exist composites $a_k$ and $b_k$ that have exactly $k$ prime factors and are such that we have $a_k-b_k=1$.
Is my belief true? Is this known? What is known about all of this and similar problems? Can someone find solutions for some larger $k$´s?
Examples exist at least for $k=1,2,...11$ by this list .