I guess we will only able to merge neurons of the same kind : neurons intraLedger merged will generate a new one itself intraLedger, neurons extraLedger merged will generate a new one itself extraLedger.
Plus, I guess neurons created with Internet Identity will not be mergeable with those created using seedphrase, but only with one itself created with II.
See ?
But I let @jwiegley be more specific or correct my assumptions.