doublesig.sol

// SPDX-License-Identifier: MIT

pragma solidity ^0.6.12;

contract DoubleSig {
    struct PendingTransfer {
        address address_to;
        uint amount;
        bool is_owner1_approved;
        bool is_paid;
    }
    
    PendingTransfer[] public pending_transfer;
    address public owner1;
    address public owner2;
    
    constructor (address owner1_, address owner2_) public {
        owner1 = owner1_;
        owner2 = owner2_;
    }
    
    // Anyone can deposit ETH to this smart contract via this function
    function deposit() external payable{}

    // owner1 or owner2 can initiate a transfer of ETH. Return an incremental unique id per initiate.
    function initiate(address to, uint amount) external returns (uint id){
        require(msg.sender == owner1 || msg.sender == owner2 , "caller must be an owner of the system.");
        
        PendingTransfer memory transfer_txn;
        transfer_txn.address_to = to;
        transfer_txn.amount = amount;
        transfer_txn.is_owner1_approved = (msg.sender == owner1);
        transfer_txn.is_paid = false;
        pending_transfer.push(transfer_txn);
        
        return pending_transfer.length-1;
    }
    
    // another owner can approve the unique id of an initiation. The transfer will happen here. 
    // Revert on invalid id or if caller is not the other owner.
    function approve(uint initiate_id) external {
        require(msg.sender == owner1 || msg.sender == owner2 , "caller must be an owner of the system.");
        require(initiate_id < pending_transfer.length, "invalid id");
        require(!pending_transfer[initiate_id].is_paid, "This request has been paid");
        
        bool is_the_other_owner = pending_transfer[initiate_id].is_owner1_approved != (msg.sender == owner1);
        require(is_the_other_owner, "this owner already approved it");
        
        address payable address_to = payable(pending_transfer[initiate_id].address_to);
        address_to.transfer(pending_transfer[initiate_id].amount);
        pending_transfer[initiate_id].is_paid = true;
    }
    
}