It seems like you want two moving images side by side ... two ends of a phone coversation. Fine .. but what I don't understand is your refrenence to still images.
The split screen is easy enough with two videos ... but, again, I don't know what or where those still images you mention fit it.
PS ... rather than re-post ... you *can* edit your previous post to fix a typo or change or add anything. Look at the blue text at the bottom right in the same box as your message. You'll see an two additional options that are not in the other messages (from other folks) to edit. You have to logged-in to see those extra options ... edit or delete.
Under effects, cookie cutter. Use Picture-in-picture and apply it to the video in the upper layer. You'll have to play with the settings to get it to your liking. Also you'll have to apply it frame by frame
Or put the two videos on two separate tracks and use track motion for each track. You can position them, size them, move them around to your heart's content.