Multi Target Syntax
    You can also combine several target definitions in a mathematical expression. Any syntactically correct expression that the Perl interpreter can evaluate to will work. An expression could be used, for example, to aggregate both B channels in an ISDN connection or to calculate the percentage hard disk utilization of a server from the absolute used space and total capacity.  
 
    Examples:
 
 Target[myrouter]: 2:public@wellfleetA + 1:public@wellfleetA
 
 Target[myrouter]: 1.3.6.1.4.1.999.1&1.3.6.1.4.1.999.2:public@mydevice /
     1.3.6.1.4.1.999.3&1.3.6.1.4.1.999.4:public@mydevice * 100
 
    Note that whitespace must surround each target definition in the expression. Target definitions themselves must not contain whitespace, except in interface descriptions and interface names, where each whitespace character is escaped by a backslash.
 
    MRTG automatically rounds the result of the expression to an integer unless RRDTool logging is in use and the gauge option is in effect for the target. Internally MRTG uses Perl's Math::BigFloat package to calculate the result of the expression with 40 digits of precision. Even in extreme cases, where, for example, you take the difference of two 64-bit integers, the result of the expression should be accurate.