Cupboards have a weird overlapping mechanic that gives rights to the owner of the cupboard that was placed first. The elevation likely prioritized your TC over there's.
Not sure why this overlapping was implemented but I had it confirmed by Facepunch as an intentional feature.
There was never a share option for them. I may consider adding locks to them giving the new farming update but I'll be closing this as this is more of a suggestion.
We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.