They provide xpubs and a wallet configuration file that has all the needed data to recreate the multisig wallet. You can't access the unchained seed but you can have them sign txs with it.
Ok so basically you have 2 of the 3 keys. I would do it the way I described above and I would ask them for suggestion for the splitting that you wish to achieve.
Since you trust the company, you can let them indicate the best ways to achieve your goals. That's what they do after all, they offer collaborative custody, so they may have some ideas.
Have you also checked Casa, or any other company? So that you can spot the differences between the two?