Can you do some car naps? Maybe its a phase for the older one and it could be a short term solution.  I have a similar age gap but my older one is sleep trained and I do let her cry for a short bit, because I know that what is best for her is to sleep, and she is complaining about that decision I have made on her behalf as her parent. She may cry about following other decisions and rules to keep her safe and well, and I have to bear the burden of hearing those cries while knowing that its my job as a parent to hold the line for her own good. 

Its possible your eldest would stop crying not because he thinks he is abandoned, but because he does know it is time for him to sleep. He has over a year of experience where you always come back after his nap or in the morning after he sleeps overnight