How can i create two mailinabox servers with same domain name

I have two servers and I would like to have mailinabox in them. I also want them to be able to send via the same domain ( . Please how can i do this without having spf and dkim issues

Only ONE of the two will actually receive email for, correct?

Could you also elaborate on why this setup would be desired? There may be a simpler solution depending on what you are actually attempting to achieve.

in case one server is down i would want to use the other.
I have actually set them up using two different servers but the issue i am having is that the dkim is failing but spf is working