This is the official Purrs carrier (well kind of, lots of us on here have them
). It opens like a clamshell on both sides so is really easy to get the cats in and out. There's a flap which is a storage pocket and a flap that you lift up that has a stroking hole, although be careful while they are small as they can poke their heads up through it!
http://direct.tesco.com/q/R.100-1535.aspx £14 from tesco - I think you can order online and collect in store at some stores now aswell to avoid paying a delivery charge.
Definitely get two carriers rather than one large one, my two shared a carrier while they were little which was fine because they didn't weigh very much. When I took them to get neutered at 6 months they went in separate carriers, when I collected them the nurse had put them both in one carrier (she said she thought it looked like a double buggy
). I let them stay together as they had just been knocked unconscious and had body parts removed so I thought it might be comforting for them - I nearly did my back in getting them to the car it was sooo heavy! They're much heavier now that they're a year old so I think it would be impossible to carry two in one basket without giving yourself a hernia!