Can a Antivrius be used for two machines by mapping the hard drive of ther other

Yeah use 2 antivirus packages.
given the ammount of time it would take to pick up an infection on one harddrive being scanned from the other machine you are better using 2 antivirus packages, otherwise your system could have its entire registry changed by a virus before the second machine picked it up. Saying that though it raises the posibility of using a paid solution like kapersky on one machine and setting it to  the harddrive of another machine, and using something free like avg on the second machine then for real time protection.
2 different antivirus packages would provide a better virus detection rate.