  Quantum cascade lasers are proving to be instrumental in the development of
compact frequency comb sources at mid-infrared and terahertz frequencies. Here
we demonstrate a heterogeneous terahertz quantum cascade laser with two active
regions spaced exactly by one octave. Both active regions are based on a
four-quantum well laser design and they emit a combined 3~mW peak power at 15~K
in pulsed mode. The two central frequencies are 2.3~THz (bandwidth 300~GHz) and
4.6~THz (bandwidth 270~GHz). The structure is engineered in a way that allows
simultaneous operation of the two active regions in the comb regime, serving as
a double comb source as well as a test bench structure for all waveguide
internal self-referencing techniques. Narrow RF beatnotes ($\sim$ 15~kHz) are
recorded showing the simultaneous operation of the two combs, whose
free-running coherence properties are investigated by means of beatnote
spectroscopy performed both with an external detector and via self-mixing. Comb
operation in a highly dispersive region (4.6~THz) relying only on gain
bandwidth engineering shows the potential for broad spectral coverage with
compact comb sources.
