We will discuss some aspects of the quark system consisting of two heavy and four light quarks in the case of two light flavors of equal mass. As an application, we will consider the low-lying Born-Oppenheimer potentials as a function of the heavy quark separation. Our analysis shows that the ground state potential is described in terms of both hadro-quarkonia and hadronic molecules. Additionally, we will consider the simplest string configurations including the five-string junctions and their implications for the system, particularly the emergence of composite quark objects different from diquarks and the process of junction fusion. We will conclude by making general comments on doubly heavy quark systems.