math question

[Anthony Palombella]

Hi, I have a math question that's been bugging me for a while, and none of
the textbooks I've looked at have been able to help.

I have a collection of specific activity data, each with an SE.  For 
duplicate assays of the same enzyme, how do I get an SD or SE for the
average of the two specific activities?  In other words, how do I go from

A1 +/- SE1, A2 +/- SE2

to an average activity with its own SE?  Any help is appreciated.  If the
calculations are too complex to write out here, pointing me in the direction 
of a good reference would be fantastic.  

TIA,

Tony