
This is because the younger one thinks he is all grown up now and can throw his weight around. Think of it in the terms of a human teenger male and his father, usually there is a period of rebellion which evenutally settle.
Having talked about this and expereinced this younster growing up and testing the adult hierachy thing (got a two year old bitch of a larger breed to yours going through this a few weeks ago) have always found being the one in charge works best. If anything kicks off both get told off equally (we are often not expert enough to work out the true dominant individual)m this basically reafirms that no need to woirk out who is boss between them, as you are boss.
Af friend has two males and the yougnert one at 2 1/2 has on occasion overstepped the mark with the older and a tiff ensued. She has told the pair of them off, anf the next things whe knows they are comiserateing with each other for having received a dresing down by the 'boss' :D
Remove anything that is the cause of tiffs when they blow up, a bit like with kids, of you can't play nicely tehn take away the toys.