I've carried as many as three. It's a bit of a nuisance to haul them all out for the scanner, and the TSA might well swab them down for explosives; but there isn't any rule against it.
If you have a dark beard or something, you might want to leave a few extra minutes to get through security; but it really ought not to be a problem.(Which is funny, since three laptops carry enough Li-ion batteries to make a right mess, and they go right past, while the TSA drones are busy telling old ladies and six year olds that their 4oz bottles of lotion are some kind of threat to the aircraft. Ah well.)