I had baby no 1 (the easy one!) at 35 and baby no 2 (the harder one, definitely!) at 36 (well, just before my 37th birthday if we are getting technical!). Two under two is difficult at times, and I think being older also can make it harder. But I wouldn't change anything. Two close together get on really well and there are other advantages too. Only you know what is right for you, but I don't think 2 under 2 at 40 should be ruled out. If you want to do it, go for it.