Other people have already covered this, but for what it's worth here's how I would do it.
(I stand to be corrected, but this is a job for Photoshop not Lightroom). There are various ways of doing it but try this:
Open one of the pics in PSD, size the image to what you want, now add enough canvas size to fit the second coin pic next to the first. Open the second coin pic, select it, add a new Layer to your Pic 1 file, paste pic 2 into the second layer of pic1. That's a start. Now you need to make selections around the coins so you can have the same background colour. There are various ways of making the selections. RTFM !
(Apologies if someone has a better way of doing this... I use Affinity nowadays, but I still have ancient PSD CS3).