Your solution is actually pretty close, but you want to use an OR gate instead of an AND gate. In general, the way to solve this sort of thing is to write out a truth table for the desired system. In this case, treat each slave TX in isolation. You want the slave's TX line to be high whenever the slave enable line is high (just for now, make it an inverse enable line, so !SLAVE_EN), and you want it to follow the TX line whenever the enable line is low. I don't know how to make a table in this editor, but if you make one up with that description, you'll find that it's a simple OR gate.