They better be pretty close together. MIRVs have a "target footprint" that is not very big....about the size of an LCC and ICBM wing.
http://www.russianspaceweb.com/r36m_family.htmlR-36M UTTKh (R-36MU)
On August 16, 1976, the Central Committee of CPSU and Council of Ministers USSR issued a resolution authorizing the modernization of the R-36M missile designated UTTKh. A primary goal of modernization was further expansion of the missile's strike range and the increase in the survivability of the vehicle and its launch complex under conditions of the incoming nuclear attack. According to the official specifications, a single R-36M UTTKh missile could strike 10 various targets, including large population centers, as well as small military targets, spread over the area of 300,000 square kilometers...
Those warheads were 20 Mt. I don't see any purpose in putting 10 20 Mt. warheads in a small footprint.