I thought I posted a response to this already lol... Anyway, as I think I said it depends on the horse. A lot of people will tell you flat no. I train my horses to lay down, so sometimes I am low to the ground with them, but I never have both knees on the ground. Always be ready to move. However, as everyone else has mostly pointed out, it isn't wise to make yourself smaller to the horse. They will take it as a submissive gesture in the least, possibly even toss you or run you over. While I don't personally care for the "It's livestock" view myself, a certain degree of awareness is needed. You get too comfortable, and even the most well mannered and friendly horse can seriously injure you. I treat my horses like family members, but I haven't ever even knelt in front of my 10 month old foal without him laying down. It's just not a good idea.