That means you can only use one of them. When you occupy either one, the other one gets disabled.
Think of it as having one table and two chairs on opposing sides of the table, and someone saying sit in any chair you like. You can choose either one, but obviously you can't sit on both chairs at the same time.
Yes it doesn't matter if you connect SSD or HDD, but you probably have 3-5 more SATA ports on the motherboard to connect 2.5 SSD to.
And here an Asus 1440p with IPS and Gsync and 165hertz
Link:https://www.newegg.com/Product/Product.aspx?Item=N82E16824236660&cm_re=ROG_Swift_PG27UQ-_-24-236-660-_-Product