  Recent numerical simulations have suggested the possibility of forming double
intermediate mass black holes (IMBHs) via the collisional runaway scenario in
young dense star clusters. The two IMBHs formed would exchange into a common
binary shortly after their birth, and quickly inspiral and merge. Since
space-borne gravitational wave (GW) observatories such as LISA will be able to
see the late phases of their inspiral out to several Gpc, and LIGO will be able
to see the merger and ringdown out to similar distances, they represent
potentially significant GW sources. In this Letter we estimate the rate at
which LISA and LIGO will see their inspiral and merger in young star clusters,
and discuss the information that can be extracted from the observations. We
find that LISA will likely see tens of IMBH--IMBH inspirals per year, while
advanced LIGO could see ~10 merger and ringdown events per year, with both
rates strongly dependent on the distribution of cluster masses and densities.
