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With the Colossus and Viper being part of the same robotics bay, it just makes sense that they should be able to "auto switch" for defense.
In other words, with an auto switch feature on the defensive base, if the Viper/Colossus is taken out then the Robotics Bay would automatically switch to the other unit.
The defending base would be responsible for ensuring health of the units before defense. Also, this would make a great opportunity for automatic regeneration boosts for both units!
General of Rage!