2 different numbers under one message thread?

I have an iPhone 5s and its software is up to date but I'm having the hardest time with my iMessages. Basically, I have two conversations from two completely different phone numbers (two different people) listed under one message thread, under one of the phone numbers. When I open the thread, I see texts I've sent to one number then as I scroll up I see texts that I've sent to another number. Basically iMessage combined two separate conversations into one. I've tried resetting my phone a bunch of times and nothing seems to work. I don't really want to have to delete the entire thread cause I'll lose two conversations at the same time. Help, please!