If you can get a good fit, with some aquarium glue/sealant/silicone glass would look better if you can drill it without breaking it, plexiglass is just easy to drill (normal drill works). My dad worked with glass for 20 years and said he'd help me when I wanted to do a divided tank.
If you put plants/decorations up against the divider to break their constant view of eachother that would be best. I put java ferns up against my dividers and they rarely flared at eachother, sometimes one will just ignore the other and one will go nuts lol. I've never seen them "ram" but I have heard of them jumping the divider >__<