Typically one becomes cathodic and one becomes anodic, based on the relative nobility of the two metals. Therefore only one gets eaten away.
Things become more complicated if there are three or more metals of different nobility in proximity to one another within some manner of electrolytic solution, or just moist earth, or if there are stray electrical currents from other sources that aggravate existing galvanic currents, and so on and so forth.
This can be a very complex subject...
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
And to toss in more variables there is the area ratio and the conductivity of the environment.
All aqueous corrosion is electrochemical, so when a stainless steel pits it is because some areas have become anodic to other areas on the same piece of metal.
If both materials are fully corrosion resistant in your environment then there is little risk of galvanic corrosion.
In your example, for nearly all cases the carbon steel would see accelerated attack.
