I think this is very true. I'm researching writer and artist George AE Russell for my PhD who was best friends with W.B. Yeats but they had huge grievances and rivalries between them. Yet AE admitted that ‘I am always fighting with him [Yeats] but if I hadn’t him to fight with it would make a great gap in my life.’
An author commenting on both summed up their relationship which could equally apply to Don and Glenn as '...the antagonism that unites dear friends'.