I have executed a task with bitbake -c task1 option. when this task is executed task2 also needs to be executed. How can I make sure that task2 will be called in bitbake. And also I have a dependency to run the task2 as independent task with bitbake -c option.
Bitbake tasks are the consecutive chain of functions with certain hierarchy. You cannot execute do_install before do_compile, or in place of do_compile, or without do_compile. Such ability is absent as it doesn't have much sense.
bitbake -c task1 means "perform everything that task1 demands before execution, execute task1 and leave". So there is no way for executing task2 that goes after task1 by command bitbake -c task1.
In my opinion -c argument should be used rather for debugging, than for work. Ideally you have to describe all relations between tasks and initiate build by image names, not individual tasks. So it seems like you have chosen the wrong tool for your purposes. What are you trying to do?