You are on one ticket issued by KE, you follow signs for TRANSFER at the airport. Korean Air might issue a Jin Air pass on their paper, but at the gate in Seoul - make sure you get the Jin Air boarding pass on their paper stock. IT's a weird thing even Korean Air does (like if you have a Delta BP for a Korean Air flight, they make you change it at the gate for their own paper). If you are following transfer signs, you are not entering the airport - so you do not pass immigration and enter Korea. Even if you only get one boarding pass, KE will check your bags through to Jin Air. They're the same parent company and they interline. AND you can visit the transfer desk in T1 OR the gate itself to get your Jin Air boarding pass anyway. You'll be totally fine.