So I'm soon to be DMing a Faewild adventure and I had an idea for a magic weapon. Essentially, an oversized pair of hedge trimmers/giant pair of scissors. They'll find each blade separately, functioning as just a +1 longsword. However the two can combine, shifting the blades to the outside edges, forming a greatsword. 

Any ideas for fun mechanics to give this thing some flavour? 

Additional damage to plants feels fitting.

Yeah, +1/+3 against plants is cool.

I was wondering about some kind of shear action. Like, if they hit with an attack, they can use a bonus action to make a strength check, maybe against the targets con save? If they succeed, they deal extra damage, or maybe cause damage over time?

Something to represent stabbing the blades in and wrenching them open.

I would avoid damage over time however, it sucks in D&D. For one thing it’s a pain in the neck to track. For another thing creatures are at full power until they drop in D&D, so it’s better to do all the damage up front.

A Str check Vs a Con save could work, but it might not be necessary depending on how you set it up. You could make it so that every time the wielder gets a Crit they can spend their bonus action to make a single attack with advantage.