Can Anyone tel me how to do butter cream in two colours gradient while doing frosting like this.i really need help next week i have to make a cake for my sister in blue and pink. this is just a cupcake.i wanna do a buttercream frosting 

Here is a tutorial, it looks like the icing in your pic is SMBC. hth
 

thanx @enga. i think this is it.

You can also use two different colors of icing bullets, that way you don't have the problem of evenly loading the bag half and half.  If you google icing bullets you should be able to find that. 

I just did something similar with cupcakes that I made with my kids. You can use the method where you paint the stripes of color on a piece of saran wrap, then put white frosting on top (search on here for frosting plug) or you can just put your two colors in one pastry bag and pipe it on that way.