I think this is a terminology thing. I paid for two sets of cabinets and paid a 10% upcharge to have them stacked. What I got looks like one integrated cabinet -- no visible seams between top and bottom, and about 1/4" between top and bottom doors (full overlay). I don't know what's under the sides and face frame (one or two cabs?), but it looks like one. My cabs are Starmark custom, but I think they're considered semicustom. (Honestly the custom/semicustom distinction confuses me.)

There's only a seam and molding if you are using partial overlay cabinets. It doesn't show with full overlay cabinets as long as a full height decorative side panel is field applied.

You can have two cabinets "combined" as one. This would have no seam on the side. Some semi custom mfg will do that, the upcharge varies. Some also offer a standard "stacked cabinet with no seam on the sides. The top sectioon in that case is a standard fixed size for each mfg. Some semi custom brands will "relocate the intermediate rail" to change the size of the opening. Other brands may require you to order the cabinet from their pricier line which might be custom or what I call "almost custom"
I know that Showplace offers standard stacked, combined, and RIR since I sell them. I believe Shiloh has something suitable? And maybe Medallion? Likely others can offer more brands.

Thank you! :)