I would suggest using CSS/SASS and overlapping with "position:absolute;" and then adjusting the position, can't really tell what your use-case is so it's hard to give you a solution.
You can also try using "float:left;", that could work right off the bat.
However I would recommend not overlapping two images, it can be very tedious and fiddly to set something like this up. Have you tried overlapping them outside of Mendix with something like Photoshop and then displaying just one image?
If you want to add two items in one table cell, you should first add an container (div):
Why? Because... Mendix doesn't support it otherwise.