Personally I would ensure that both t-shirt and trousers were both in place before venturing downstairs. However, after that, my decision would depend on whether I was tucking my t-shirt into my trousers or not. In practice, I only tuck t-shirts in when I'm wearing a jumper on top. If I'm not tucking in, it would be trousers first. If I am, it would be trousers second, so that the tucking in happens automatically.