A bit of a meandering answer to the question, and to answer a side question: fundamentally what is a scaling solution?
OP_CTV does not increase the block space. Transactions using generally will use more block space actually (although it need not be a large amount), and relay can be optimized to equivalent.
But scaling is more than just a block size ...
Based on your example it looks like you are signing the same sighash twice, so it only looks like one key signed.
This example might help: https://github.com/magmahindenburg/MultisigExamples/blob/master/src/se/moar/bitcoin/multisig/MultisigPresentation.java
// Sign the first part of the transaction using private key #1
Sha256Hash sighash = spendTx....