my wife and I both have iphone 5
thinking of buying unlocked 5S for both of us, finish the last year of our contract and then go to t-mobile
once i put my AT&T sim into the unlocked 5S, can AT&T or apple lock it since i still have another year with AT&T?
As long as it is a Apple store purchased unlocked phone, then no. It's your phone, and your carrier has no claim over it beyond requiring it/you to adhere to the terms of service. If you break them, then they could terminate your line, but not demand the phone.
As far as I understand, even the 5 you have was yours from the start. I don't think any contract is secured against the phone supplied, so that 'free' phone is really free, only you have entered into a legally binding contract to pay them way more than the cost of the phone over two years. Fail to do that, and they demand the money, not the phone. The only hold the carrier has over it is that they won't agree to unlock it if you aren't in good standing with regards to your payments, use etc..
David
