There are servers with rack-mounting kits that specifically are designed for center mounting. Usually they have ears that mount to the rails at the center, then another set of ears that will mount to the front of the rails so you can screw the ears of the server into it, so they don't slide forward as you are pushing cables in the back.
However, the servers I've seen that have these rails were years ago on Supermicro servers with the sturdy ball-bearing rail kits. The servers in the last several years have all been missing this capability. So mostly you'll just need to check the capabilities of the servers.
Another option I've done in the past was to get 2 2-post kits and use them for the front and back rails making my own 4-post rack. I used a double layer of plywood to bolt the bases into, then I used a cabling ladder bolted to the top of both and then mounted to the wall to stabilize it, say for an earthquake. Worked very well and was effectively a 4 post rack.
If you don't need maximum density, front and rear mounted shelves and sit the server on those. But that won't be something you can screw into, and if it tips any it could all come tumbling down, but also you may not be able to get servers in the mounting space between the shelves, which could half the density.