Let's pretend you have a fighter/druid with a 17 Con, ok?
When this pc levels-up as a fighter he will get 1d10(dice roll) PLUS 3(Con bonus) divided by 2(number of classes).>>>(1d10+3)/2
When this pc levels-up as a druid he will get 1d

dice roll) PLUS 2(non-fighters can get a MAX of 2 bonus HP) divided by 2(number of classes).>>>(1d8+2)/2
Clear as mud, eh?