redshift serverless

As an administrator you first create the two users: user1 and user2

Then you create the schemas with authorization for each user respectively, example create schema schema1 authorization user1;

Below statements will do what you are looking for:

create schema schema1;
create user user1;
grant usage on schema schema1 to user1;
grant select on all tables in schema schema1 to user1;

create schema schema2;
create user user2;
grant usage on schema schema2 to user2;
grant select on all tables in schema schema2 to user2;