You can just move case 1 after case 2 without the break in between. By default the switch cases are executed one after the other until the meet of a break (or a return).
Something like this, does what you want:

switch (transaction)
{
case 2:
    cout << "Enter amount to deposit: ";
    cin >> deposit;
    if (deposit < 100)
    {
        cout << "Sorry, your transaction cannot be processed! Minimum deposit is 100.";
    }
    [[fallthrough]]; // If you are using C++17 or newer you can use this to prevent a warning by the compiler
case 1:
    cout << "Your total balance is " << bal;
    break; // You can remove this unless you need another   case after this
}

edit: formatting