IS-IS routers operate on each routing level independently. For example if there are two Level 1 IS nodes, they will only form a level 1 adjacency. If there are two Level 2 IS nodes they will only form a level 2 adjacency. If there are two Level 1/2 IS nodes, they will form a Level 1 adjacency AND a Level 2 adjacency.
IS-IS routers not only maintain separate adjacency’s for each level of routing they also keep separate LSDBs for each level of routing as well. Contents of these LSDBs are shared over their respective level to their adjacency’s via their respective LSP advertisements. For example Level 1 LSPs will inform other Level 1 IS nodes about the advertising routers connected ES nodes. The Level 1 LSDB is maintained by these types of LSPs alone. Level 2 LSPs do the same for Level 2 adjacency’s and Level 2 LSDBs.