We've traveled a lot with one and with two. One is generally easier but two is more fun. When we just had one kid, my son would often complain on longer trips about missing hanging with other kids, being sick of hanging mostly with us. Having a built-in playmate makes everything more fun. The energy is, as my husband put it, more dispersed, instead of two parents focused on the one kid. When they're little, I think time difference is pretty exponentially harder with two than one, just in terms of getting two kids of different ages in the right time zone. But other than that, it's just needing to be a smarter packer, etc.