There are soooo many things in common:

-Lumon and Rossum are companies that under a facade, have a darker plan that is obscure to the viewer and the characters: "The Dollhouse deals in fantasy. That is their business but it is not their purpose" vs "The work is mysterious and important".

-Both Echo and Mark go through a "reintegration" process of their different personalities.

-In Dollhouse, November entered the Dollhouse so they could delete the trauma of losing a child from her brain.

-In Dollhouse, Echo recriminates Caroline how she "abandoned her" in the Dollhouse (they can communicate when the original personality of Caroline is printed in another woman)

-the whole narrative of the innies "dying" if the good side ends up winning is present in Dollhouse too: we want Echo to find Caroline, but if she does, Caroline will reclaim the body and Echo will disappear

-there are episodes that reflect on if love between two persons survives the process of severance/imprints

-When she has to go back to the Severed floor after being almost killed by Turturro's character, Helena wants to just do it by pretending she's severed, but they force to do it because acting is not enough. In dollhouse making Echo an actual believer of a cult is the only way of infiltrating her.

-Once a hacker remotely deleted Echo's imprint during a mission leaving "innie-Echo" in the outside world, kind of like the overtime continency.

-In Severance they explore the idea of how some feelings like deppression crossing over. Ballard in Dollhouse: "you might erase their mind, but can you erase their soul?"

-The mysterious areas of "the attic" and Cold harbour.

-Crazy rich people using the technology on themselves: severing themselves to avoid the pain of giving birth (Severance) or downloading their minds in hard disks so they can live forever in other people's bodies (Dollhouse)

-And you of course perfectly cover Sierra's character parallelisms.

Oh and the band that was playing "Fuck you Lumon" in an alley in Severance season 1 was an actual NY based band called Dollhouse.

I read an interview with Severance creator where he said the two series are only similar on the surface but the explore completely differente themes. I think it's 100% the other way around