… from nurseryworks that looks modular, I wonder if it’s bunkable, like you can buy one bed now and add the second later …

Duet Bunk Beds by truck product architecture for nurseryworks:NurseryWorks     *  duet bunk bed

* • stack or unstack beds
* • two cabinets
* • six open cubbies
* • three drawers under bottom bed
* • upper cabinet is accessible from top bunk
* • cubbies become a separate shelving unit when beds are unstacked

As always, the prices are “modern” too:
$2,490 with drawers or trundle
$1,200 bottom bunk with drawers or trundle