  We study resonant dipole-dipole coupling and the associated van der Waals
energy shifts in Rydberg excited atomic rubidium and potassium and investigate
F\"orster resonances between interspecies pair states. A comprehensive survey
over experimentally accessible pair state combinations reveals multiple
candidates with small F\"orster defects. We crucially identify the existence of
an ultrastrong, "low" electric field K-Rb F\"orster resonance with a extremely
large zero-field crossover distance exceeding 100 $\mu$m between the van der
Waals regime and the resonant regime. This resonance allows for a strong
interaction over a wide range of distances and by investigating its dependence
on the strength and orientation of external fields we show this to be largely
isotropic. As a result, the resonance offers a highly favorable setting for
studying long-range resonant excitation transfer and entanglement generation
between atomic ensembles in a flexible geometry. The two-species K-Rb system
establishes a unique way of realizing a Rydberg single-photon optical
transistor with a high-input photon rate and we specifically investigate an
experimental scheme with two separate ensembles.
