coinb.in supports creating/signing transaction with multisig address
first, you need Redeem Script to retrieve utxo from multisig address
then at signing you need to do it twice (if 2 of 3) consecutively with each privkey

unsigned rawtx + privkey1 = partially-signed rawtx
partially-signed rawtx + privkey2 = multi-signed rawtx

but HCP has explained, mempool-conflict = attempted "double spend"
@OP if you are still in doubt, check your multisig address balance on any explorer
or tell us the address, so we can check and give advice on the matter