It appears there may have been a misunderstanding of your requirements. Are you looking to establish a nested VPN connection—specifically, running a second VPN tunnel through an already active first VPN tunnel?
If so, this "VPN-over-VPN" configuration is not natively supported in the current firmware.
However, a workaround exists that can achieve a similar effect. You can find the technical details and implementation steps in this community discussion:
We have also submitted this to our product team as a formal feature request for evaluation. We hope to include native support for VPN chaining in future firmware versions.