Here is the Twitter response I received from T-Mobile:
"thanks for your patience here while I dig through my toolbox. I would recommend submitting both rebates for both offers. I don't have any information saying they can't be combined but I also don't have any information saying they can be combined so it doesn't hurt to try! The worst that is going to happen is that you'll be denied the rebate for the Tax Season Smartphone offer which would be the tax amount. I just want to make sure you get the rebate for the Samsung Superphone offer!"
It appears there are no publicly stated restrictions from stacking them... therefore, both "should" be honored.
I just took a look at the both of those offers and there is nothing that says those 2 specific offers cannot be combined. I did find out that 17Q1SAMTRADE (the free device/$100 for edge) cannot be combined with our 17SWITCHER offer ($150 rebate for porting in between 1/5 - 2/28) but there is nothing that says 17Q1SAMTRADE cannot be combined with 17DEVICECREDIT.