You can set up your merger/splitter using two standard splitters and two standard mergers: split each of the two inputs, then merge one output from each splitter into the output. That will give you 2x900 into 2x900 rebalancing.
This can be generalized into any M to N rebalancing, essentially first split each of the M inputs into N branches equally, then merge one branch from each input into each of the N outputs.