𝐌𝐄𝐑𝐆𝐈𝐍𝐆 𝐆𝐈𝐅𝐒

So this is the most basic and important part of creating a gif banner because without this skill, you won't be able to create one cohesive image. Sometimes you will find that it is easier to do than other times but this is mainly to do with your gif selection as the process remains the same at all times. It may seem awkward and confusing at the beginning but it will come like second nature to you after a while. 

𝐅𝐈𝐑𝐒𝐓 𝐒𝐓𝐄𝐏
𝐒𝐄𝐋𝐄𝐂𝐓𝐈𝐍𝐆 𝐘𝐎𝐔𝐑 𝐆𝐈𝐅𝐒

So before you get to merging your gifs together you need to make sure that your two gifs have a similar background or colouring and that maybe even the facial expressions/emotions match in some way. This was ultimately make it easier for you when it comes to blending and give you the best end result. 
For example...

𝐒𝐓𝐄𝐏 𝐓𝐖𝐎
𝐌𝐀𝐓𝐂𝐇𝐈𝐍𝐆 𝐓𝐇𝐄 𝐅𝐑𝐀𝐌𝐄𝐒

Once you have opened your two gifs into Photoshop (File + Open) then your first step is that you should open up the gifs Timeline. You can do this by clicking on Window in the top tab of photoshop and clicking on Timeline. Now you should be able to see each frame of the gif at the bottom of the screen that match up to the number of layers you have in the bottom right corner. 
Like so...

Now that you have this on both of your open gifs, you need to match the amount of frames so that they correspond. So, for example with these two gifs, the one of Synnove is 61 frames while the one of Tommy is 24. So you need to delete 37 frames from Synnove's one. And you can do this quickly by clicking on the first frame then holding shift and clicking on the last one, this will select all of the frames you want to delete, and then all you need to do is click on the rubbish bin on the bottom of the screen. You can also group all the layers (bottom right corner) together into one folder to make things more organised and easier to look at by using the same method of holding shift and selecting the first and last layer. 

𝐒𝐓𝐄𝐏 𝐓𝐇𝐑𝐄𝐄
 𝐋𝐀𝐘𝐄𝐑𝐈𝐍𝐆 𝐆𝐈𝐅𝐒

If you look on the Timeline section, on the right hand side, you will see three horizontal lines on top of each other, you now need to click on this and choose Select All Frames 

Now click on it again and choose Copy Frames. Now go onto your other gif and click on the same three horizontal lines and choose Select All Frames, and then Paste Frames. Now a little box should pop up in the middle of your screen that looks like this...

You need to make sure that you select Paste Over Selection and tick the Link Added Layers

𝐒𝐓𝐄𝐏 𝐅𝐎𝐔𝐑
 𝐌𝐎𝐕𝐈𝐍𝐆 𝐆𝐈𝐅𝐒

Now you can have fun moving your gifs around each other and adjusting their sizes so that they match somewhat and also give you enough space in the middle to blend your gifs together. 
Like I have done here... 

𝐒𝐓𝐄𝐏 𝐅𝐈𝐕𝐄
𝐁𝐋𝐄𝐍𝐃𝐈𝐍𝐆 𝐆𝐈𝐅𝐒

Now comes the frustrating part. So, first, you need to convert this into a video timeline which you can do in either two ways. One by clicking on the three horizontal lines and choosing Convert to Video Timeline or by clicking on the three horizontal lines that have a line cutting down the middle, on the bottom left hand side. 

Now that you have done that, choose the group in your layers tab that is on top and add a mask by clicking on the white square with the cutout circle on the bottom right corner of the screen. It should now look like this. 

Now click on the paintbrush tool and make sure that the paintbrush is on a soft round pressure opacity and that the hardness is at 0%. Then choose the colour to be black and start slowly erasing the harsh lines and stuff in between that you don't want on your top gif. Once you have gotten a look that you like hopefully it will look something like this.  

𝐒𝐓𝐄𝐏 𝐒𝐈𝐗
 𝐅𝐈𝐍𝐀𝐋 𝐓𝐎𝐔𝐂𝐇𝐄𝐒

Typically at this stage is where I add my colourings, text and size the gif banner. For the sizing you can click on the Image tab at the top of photoshop and click on Image Size. I usually keep the width at 500 and the height at anything between 190 to 210 dependent on your ratios, but you can always use the crop tool to make some final adjustments. To save your gif, go to the File- Export - Save for Web (legacy) and this screen should pop up...

Make sure that you have selected Gif as the file type and that the rest of the options are set as shown above. Then once you are all done, click save and enjoy your final product!
(If you have any questions, don't be scared to ask!)

𝐌𝐘 𝐅𝐈𝐍𝐀𝐋 𝐏𝐑𝐎𝐃𝐔𝐂𝐓...