Security implications of guest account and database chaining on sql server?
It does not open up directly to an "attack".
It just means that any user from Database 1 (Kdb) can also access database 2 (Ydb).
What's usually more critical is, when you have users with DDL-Permissions (create views, procedures) - they will also be able to access objects in database2. Maybe even more, than plain guests can. That depends on the object owners and permissions you are planning to grant to guest(s).

Under SQL Server before 2012, there was one little security hole, described here:
Security-issue: guest-guest impersonation, but this is a very limited scenario