Software affects every aspect of your unit,
(Maps/GPS, 2D, SI, DI, Views, Depths, Scroll Speed, etc.).
Each unit usually has it's own unique software.
Just because the software says 6.50 for two
separate units, it does not mean that software from one
can be used on both units.
If you have not done so, register your two units
on the Humminbird Web site.
Call Humminbird customer service, and they may be
able to tell you what version of the software your unit(s)
shipped out with.
You can always play around with your units(s) off the water, and use
the simulator to see if they work OK.
They may both be fine.