Seven Segment Drives and Decimal Point Drive that source current to the display. On the MAX7219, when a segment driver is turned off it is pulled to GND. The MAX7221 segment drivers are high-impedance when turned off
I guess you realize that this is not relevant, since the segments are not common (in the 2xMAX7219 multiplexed setup), just the digit drivers.
What is relevant is this:
The MAX7219 pulls the digit outputs to V+ when turned off. The MAX7221’s digit drivers are high-impedance when turned off.
Now think of the internal npn transitors for sinking the digit lines. The "digit output to V+" is realized through a pullup resistor. When one MAX7219 is shutdown, the digit line of the one that is on will sink a bit more current (LED current + current through the pullup resistor from the one in shutdown mode). If the pullup resistor is 10k (let's say), the extra current is 0.1mA (5V/10k).
If it's not clear, I could draw a schematic.
Please let me know what you think.
Where is ol' Mike when you need him? 