vechain/thor

Hi again!
Can you please help me with understanding VIP-191 concept of signing delegated transactions?
I checked and tried this demo: https://github.com/zzGHzz/ThorDemo3 and it works pretty well, but it uses connex.vendor.sign('tx').request() method for singing and sending transactions.
What I need is to sign delegated transaction avoiding use of wallet. So I just want to send delegated transaction like that:

const tx = new Transaction(body);
const signingHash = cry.blake2b256(tx.encode());
tx.signature = cry.secp256k1.sign(signingHash, Buffer.from(sk.slice(2), 'hex'));
const encoded = tx.encode();

const raw = '0x' + encoded.toString('hex');
let ret = await net.http("POST", 'transactions', {
headers: { 'x-genesis-id': connex.thor.genesis.id },
query: {},
body: { raw }
});

So before sending it, I also have to sign it by gasPayer like that:
const tx = new Transaction(body);
const hash = cry.blake2b256(tx.encode());
const delegatorSigningHash = cry.blake2b256(hash, Buffer.from(origin.slice(2), 'hex'));
const sig = cry.secp256k1.sign(delegatorSigningHash, Buffer.from(skDelegator.slice(2), 'hex'));

As it said in VIP-191 docs, signature of delegated transaction consists of 2 parts:
Bytes | Attribute |Type | Optional
0-64 | senderSignature | byte[65] | no
65-129 | gasPayerSignature | byte[65] | yes

According that, how to concatenate both signatures properly to achieve raw transaction which is ready to send?
Thanks in advance.