  Emergence of gravitational wave (GW) astronomy revived the interest in
exploring the low frequency GW spectrum inaccessible from the ground. Satellite
GW observatory DECIGO in its original configuration and the currently proposed
smaller scale B-DECIGO are aimed to cover deci-Hertz part of the GW spectrum,
which fills the gap between LISA mili-Hertz and deca- to kilo-Hertz range
probed by ground-based detectors. In this paper we forecast the detection rates
of inspiraling double compact objects (DCOs) and the unresolved confusion noise
from these sources in DECIGO and B-DECIGO. In the context of DECIGO we use, for
the first time, the population synthesis intrinsic inspiral rates of NS-NS,
BH-NS and BH-BH systems. We also estimate the expected gravitational lensing
rates of such sources for DECIGO and B-DECIGO. The result is that yearly
detection of DCOs inspirals for the DECIGO is of order of $10^4 - 10^5$, while
for a much smaller scale B-DECIGO they are about $10^2 - 10^4$ depending on the
DCO population considered. Taking into account that considerable part of these
events would be detectable by ground-based GW observatories the added value of
DECIGO/B-DECIGO could be substantial. DECIGO will also be able to register
$5-50$ lensed NS-NS inspirals and up to O(100) BH-BH inspirals. On the
contrary, predictions for the B-DECIGO are pessimistic: only the optimistic
common envelope scenario of DCO formation could yield a few lensed BH-BH
inspirals per year. We also address the question of the magnification bias in
the GW event catalogs of DECIGO and B-DECIGO.
