NAKIVO update with support of vSphere 6.7 update 3 was added in the custom build of NAKIVO 9.0.0 version. This information (support of 6.7 u3) is not shown on NAKIVO web site. We expect this information within next release of NAKIVO 9.1.0
Yes, that algorithm of using one storage with two transporters is possible.
But please pay attention that if second transporter will try to attach the repository, it could lead to the repository corruption.