I have a solution, hope it can help.
1. remove the outer VLAN on the downstream switch, this is easy to implement on the downstream switch, just added the outer VLAN as untagged mode. so, the outer VLAN will be removed when the packets is sent out from the interface.
2. configure 2:1 VLAN mapping on switch, map the 2 inner VLAN to the first inner VLAN, the second inner VLAN is terminated.
you can have a try.