Ethereum Solidity Payable Method Demo

MyContract.sol

import "./SafeMath.sol";

contract Constants {
    uint8 public constant BUY_ID = 1;
    uint8 public constant SUBSCRIBE_ID = 2;
}

contract BuyContract is Constants {
    
    uint256 public lastAmount;

    function buyMethod(uint8 _buyArg) payable public {
        require(_buyArg == BUY_ID);
        lastAmount = msg.value;
    }
    
}

contract SubscribeContract is Constants {
    
    uint256 public lastAmount;

    function subscribeMethod(uint8 _subscribeArg) payable public {
        require(_subscribeArg == SUBSCRIBE_ID);
        lastAmount = msg.value;
    }
    
}

contract MyContract is Constants {
    using SafeMath for uint256;

  function singleTransaction(
      address _buyAddress,
      address _subscribeAddress
  ) payable public {
      BuyContract buyContract = BuyContract(_buyAddress);
      SubscribeContract subContract = SubscribeContract(_subscribeAddress);
      uint256 halfValue = msg.value.div(2);
      buyContract.buyMethod.value(halfValue)(BUY_ID);
      subContract.subscribeMethod.value(halfValue)(SUBSCRIBE_ID);
  }

}

SafeMath.sol

pragma solidity ^0.5.2;

/**
 * @title SafeMath
 * @dev Unsigned math operations with safety checks that revert on error
 */
library SafeMath {
    /**
     * @dev Multiplies two unsigned integers, reverts on overflow.
     */
    function mul(uint256 a, uint256 b) internal pure returns (uint256) {
        // Gas optimization: this is cheaper than requiring 'a' not being zero, but the
        // benefit is lost if 'b' is also tested.
        // See: https://github.com/OpenZeppelin/openzeppelin-solidity/pull/522
        if (a == 0) {
            return 0;
        }

        uint256 c = a * b;
        require(c / a == b);

        return c;
    }

    /**
     * @dev Integer division of two unsigned integers truncating the quotient, reverts on division by zero.
     */
    function div(uint256 a, uint256 b) internal pure returns (uint256) {
        // Solidity only automatically asserts when dividing by 0
        require(b > 0);
        uint256 c = a / b;
        // assert(a == b * c + a % b); // There is no case in which this doesn't hold

        return c;
    }

    /**
     * @dev Subtracts two unsigned integers, reverts on overflow (i.e. if subtrahend is greater than minuend).
     */
    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        require(b <= a);
        uint256 c = a - b;

        return c;
    }

    /**
     * @dev Adds two unsigned integers, reverts on overflow.
     */
    function add(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a + b;
        require(c >= a);

        return c;
    }

    /**
     * @dev Divides two unsigned integers and returns the remainder (unsigned integer modulo),
     * reverts when dividing by zero.
     */
    function mod(uint256 a, uint256 b) internal pure returns (uint256) {
        require(b != 0);
        return a % b;
    }
}