Join Date: Nov 2018
Location: Istanbul, Turkey
Programs: TK Elite Plus,BAEC GGL,ITA Executive, AFKL Gold,QR Gold,HH Diamond,Bonvoy Gold,ALL Gold
Posts: 14,186
TK does check bags on two separate tickets so you would be fine during this layover.
If ATH can't issue your IST-IAD boarding pass, you could always stop by the transfer counter at IST before passport control and reclaim your boarding pass there for that leg.