So what you want to do is let tasks under an operating system start and stop other tasks. Please try to formulate your questions more clearly, otherwise people will not generally understand you.

To have this sort of feature is a job for your operating itself and will have to be accomplished by implementing a messaging system through which tasks can send requests to the OS to start and stop other tasks. What you DON'T want to do if you want to build a half serious "multitasking OS" is have tasks communicate with each other directly through registers or memory locations, or pass control directly by using any JMP, CALL or RET, without intervention of the OS. If you do that, what is the point of the OS in the first place????
So forget JMP or CALL or RET, they do not qualify for the job you have, unless you are willing to cheat and not build a real "multitasking OS"

As for the PUSH, PUSH, RET versus LJMP: they have really exactly the same effect. The processor is not impressed with you using a RET any more than by you using an LJMP instruction. If you think your PUSH, PUSH, RET has a significantly different effect than LJMP, you are in really deep trouble, and you lack basic knowledge which you'd better fix BEFORE you focus your attention on a "multitasking OS"

In general (not to Farhan personally):

I don't know whether to laugh or cry about this increasing tendency to see the 8051 as a mini PC, to confuse the need to develop your ability to code beyond JNB BUTTON,$ with MULTITASKING and the general ease with which basically unable people tend to think that the fact that there are so many Windows PC's and MP3 players around them makes it EASY to do similar things in their bedroom.