The error message indicates that the Connect attachment is already associated with another transit

gateway route table. You cannot associate the same attachment with more than one route table.

However, you can propagate the same attachment to multiple route tables. Therefore, to fulfill your

requirement of configuring a second route table for east-west traffic inspection between two VPCs,

you need to create a propagation with the Connect attachment in the second route table. This will

allow the second route table to learn the routes from the Connect attachment and forward the traffic

to the security VPC1. You also need to associate the second route table with the Transport

attachment, which is the transit gateway attachment for the security VPC1.

Reference:

Transit gateway route tables - Amazon VPC | AWS Documentation

Getting started with transit gateways - Amazon VPC | AWS Documentation

Configuring TGW route tables | FortiGate Public Cloud 7.4.0 | Fortinet Document Library