Mark input as external PSBT Partial sign

[jafri]

I'm trying to combine inputs from 2 different ledger to create to output a single UTXO

Example:

Inputs
#0   |  0.1 BTC
#1   |  0.1 BTC

Outputs
#0   |  0.18 BTC

In bitcoinjs-lib, I'd do:
A. Sign a PSBT with input 0 and output 0 with SIGHASH_SINGLE | SIGHASH_ANYONECANPAY
B. Sign for input 1 with SIGHASH_ALL

However A is not possible with ledger it seems to do a check of outputs < inputs

So I'm guessing the way to do this would be to provide the full inputs and outputs, while marking input 1 initially as external.

Is this correct? If so, how do I mark input 1 as external?

So far I have for a single payment:

    const result = await ledgerBtc.createPaymentTransaction({
      inputs: [
        [splitTx, +index, undefined, undefined],
      ],
      associatedKeysets: [`86'/0'/0'/0/0`],
      outputScriptHex,
      additionals: ['bech32m'],
      // sigHashType: BitcoinLikeSigHashType.SIGHASH_SINGLE | BitcoinLikeSigHashType.SIGHASH_ANYONECANPAY,
    });

This already throws an error if I uncomment sigHashType