Depends what you mean. On the wireless side both machines compete for the bandwidth and they do not share evenly so some gets wasted. So if you had say 54m running 802.11g. In a perfect world they would each get 27m. Still with only 2 machines it share pretty well so you could easily get 20m each.
Now all this doesn't really matter. Few if any home users have a internet connection that can really run 20m much less 54m.
In most cases it is the sharing of the internet connection itself that slows things down.
do their connection speed slow down comparing with just one computer on the wireless home network?