So far we’re thinking about this as a 1 way migration, because the hope is that eventually everyone will be on L2 and the L1 network won’t be needed anymore.
So migration is always from the L1 indexer to one L2 indexer address, just allowing to send parts of the stake at a time (as long as both sides are over the minimum stake).
Edit: @toxf1 I just published GIP-0046, that describes the migration helpers I mentioned here: GIP-0046: L2 Migration Helpers - feedback welcome!