Hi Becky.
I'd probably start off by just putting them both in and see how they get on.
They will need to stay in the pen for a good few days anyway, so they will definitely need a litter tray unless you want a very messy and soggy floor.
As there is no door so you can't get to them, you will need a way to stop them from fighting should it occur, so you should have a long handled broom or similar to put between them if it kicks off. Alternatively you can hold a towel between them or even put a towel over one of them if it does get to a fighting stage.
Chasing is okay of course as long as there is no blood, injury. You might well see some fur though, and have close watch on Mopsy's ears for any cuts. If this happens then you might want to reconsider a bond.
I personally do add a litter tray normally, but the last bond that I started, one of the buns was guarding it so we moved them onto a neutral bit of grass in a very small run so that the defensive bun couldn't do this. This seemed to work and the three buns are now very much in love. I added the litter tray back a few hours later.
I agree with Erin and actually sprinkle the pellets; using a bowl can sometimes (not always) lead to a problem if they both want to get their snouts in the small space and don't like being so close to each other whilst eating.
If you get stuck, don't forget, you can always give me a shout.
Good luck missus.