There is an issue in linking multiple accounts from Thrift Saving Plan which are divided by 'Civilian' and 'Military' type accounts.
Although these accounts use separate logins with the same username, they are separated by a different password. Our Engineering team is working on this, but currently we do not have an ETA for the resolution of this issue and recommend that you add the second account as a Manual Investment Holdings account.
Please reference this article for any future updates.