You do not write how large the current of the 6.5 VDC signal is. Different solutions may be the best depending on the current requirements, likewise if the control voltage and the 6.5 VDC must be fully electrically isolated from each other.
The simplest solution (in case they must not be fully isolated) would be to make use of a switch arrangement based on MOSFET:s. Should the requirement be at hand that the 5 VDC and the 6.5 VDC signals must be fully isolated from each other, then I would suggest a design based on so called solid-state-relays.
The are essentially opto-couplers but with a power MOSFET as the output. I assume that such exists that exhibits the quality of not needing knowledge of which of the output's must be connected to the most positive and/or negative signal. I.e. they operate as a relay but consumes much less power and does not suffer from contact bounces or mechanical fatigue problems.
/Pim