I built Windstad Manor and Valdimar was my Steward but I asked him to be my follower. Now I would like another Steward to watch over my house while we are away. Is this possible and if so how do I go about getting another one without losing track of Valdimar?

You can only have one steward for each house, Valdimar is your steward for Windstadt Manor and will be till the day he dies, having him as a follower makes no difference to him being your steward. If you are playing on the PC you could use the console maybe or a mod, but on the Xbox 360 the only way to get a new steward is to kill Valdimar. You could have left him as your housecarl and got yourself someone else to be steward if you wanted someone still guarding the house. I would not worry about it, nothing is going to attack the house while you are not there, and if your character has no spouse there is no one for even a random kidnapping. And the kidnapping would happen even if you had a housecarl and steward guarding the place.