Sql-server – “Login failed for user…” with cross-instance access on same server in SSIS package

I have 2 SQL Server instances running on the same server.
Lets call the instances Instance_A and Instance_B and the server Svr0045.
Both server use the same service accounts (a service account is bound to the server, not the instance, so they all share the same service accounts). For simplicity sake, this service account is named Svr0045_SQLSvc.

I have an SSIS package that is running on Instance_B under the service account which tries to access Instance_A using an OLEDB Connection Manager set to Windows Authentication.
I say "tries" because i get the "Login failed for user 'Svr0045_SQLSvc'" error when i run the package.
Since the service account is the same for both instances i assumed that it should work.
The service account has sufficiant rights (since it's a member of the sysadmin role), is granted connection to the instance and is enabled.

Questions:
Is this problem known / is there a reason to this?
How can i get it to work (if there even is a way)?