Given is string: dog apple orange banana
dog apple orange banana
I need to make it: monkey apple cow banana
monkey apple cow banana
That is without calling sed twice.
The following sed example should solve your problem. sed allows multiple -e switches, which allows you to replace more than one thing at a time.
sed -e 's/dog/monkey/g' -e 's/orange/cow/g'
Use ; to queue commands:
;
sed -e 's/dog/monkey/g;s/orange/cow/g'