policy.add(policy.suffix(policy.STUB({'$our ip'}), extraTrees))

Can I put more than one IP at this line to forward the queries? We have
2 authorative servers, so I want ask both of them round robin like.

Up to four addresses are supported inside a STUB "command", but the semantics isn't really round-robin - kresd prefers servers that answer faster, and there's not much "randomization" so far (I have such improvements in plans), so it may well end up using only a single one, as long as it answers fast and without errors.

Is there a way to get my desired behavior?

Config is free-form lua, so you may e.g. use techniques like in [1], except with STUB instead of TLS_FORWARD.
[1] https://www.ctrl.blog/entry/kresd-random-dns-forwarding

--Vladimir