I wonder why you think that seeing your boyfriend two times a week is not very often. I would be happy seeing a boyfriend that frequently, whether I had another partner or not. You live with your husband, see your beau twice a week, have two kids, and you are still lonely?
While I think it's perfectly fine to have as many lovers as you want, be careful if you are just wanting them to fill up some sort of lack or emptiness inside you.
One can be quite alone and never feel lonely. Yet here you are surrounded by loved ones and feeling that way. Before you pursue having another additional boyfriend, I would suggest you start examining your feelings that what you have is not enough and see where that comes from. Maybe take some time to be alone and see what emotions come up for you, and then ask yourself why. I would want to get to the root of the loneliness instead of trying to just comfort myself with a man's company.