For research work, I need to do double baking on mainnet, what is the possibility to do this, I ran two copies of the baker on two servers, but it didn't work due to the fitness too low error
I think you'd need to disconnect the second node from the network just before the second baker injects its block. In this way the block of the first node does not reach the second node. The two nodes will have (possibly) different blocks at the same level and round (to make sure they are different, you may also try to inject a transaction in the second node). After the baker injects, then you reconnect the second node.
I'm not completely sure this is sufficient.