"The MCP73842 and MCP73844 are designed for
applications utilizing dual series cell Lithium-Ion or
Lithium-Polymer battery packs. Two preset voltage
regulation options are available (8.2V and 8.4V). The
MCP73842 and MCP73844 operate with an input
voltage range of 8.7V to 12V."
Charger uses thermal monitoring to keep it all safe.
You don't have the higher voltage availanble for the charging current?
Otherwise, need to implement some MOSFETs for voltage isolation & switching.