in Photoshop CS6 you open both gifs in different tabs; in the first tab you click on the drop down list on the upper right hand corner of the Animation Timeline window and select Select All Frames; then you choose Copy Frames from the same drop down menu; then you go to the second gif tab and in the Timeline Animation window select the frame where you want to paste the Frames; click control +V and you'll be given an option to paste the copied frames either before or after the frame you selected