  Inclusive Higgs boson pair production through the mechanism of gauge boson
fusion e^{+} e^{-} -> V* V* -> h h + X (V=W,Z) in the general Two-Higgs-Doublet
Model (2HDM), with h=h^0,H^0,A^0,H^{\pm}, is analyzed at order \alpha^4_{ew} in
the linear colliders ILC and CLIC. This kind of processes is highly sensitive
to the trilinear Higgs (3H) boson self-interactions and hence can be a true
keystone in the reconstruction of the Higgs potential. For example, in the ILC
at 1 TeV, the most favorable scenarios yield cross-sections up to roughly 1 pb,
thus entailing 10^5 events per 100 fb^{-1} of integrated luminosity, whilst
remaining fully consistent with the perturbativity and unitarity bounds on the
3H couplings, the electroweak precision data and the constraints from
BR(b->s\gamma). Comparing with other competing mechanisms, we conclude that the
Higgs boson-pair events could be the dominant signature for Higgs-boson
production in the TeV-class linear colliders for a wide region of the 2HDM
parameter space, with no counterpart in the Minimal Supersymmetric Standard
Model. Owing to the extremely clean environment of these colliders, inclusive
2H events should allow a comfortable tagging and might therefore open
privileged new vistas into the structure of the Higgs potential.
