Think I've managed to sort out the problem using a custom view as shown below, it's not perfect (selection dims the buttons darker rather than lighter for example) but I'll try fixing that tomorrow. Just glad my headache is over! Thank you for your help, it lead me to a few new approaches I didn't try.
UIImage *firstButtonImage = [UIImage imageNamed:@"firstButton"];
firstButtonImage = [firstButtonImage imageWithRenderingMode:UIImageRenderingModeAlwaysTemplate];
UIButton *firstButton = [[UIButton alloc] initWithFrame:CGRectMake(0, 0, 35, 35)];
[firstButton setImage:firstButtonImage forState:UIControlStateNormal];
[firstButton addTarget:self action:@selector(firstButtonPressed) forControlEvents:UIControlEventTouchUpInside];
UIImage *secondButtonImage = [UIImage imageNamed:@"secondButton"];
secondButtonImage = [secondButtonImage imageWithRenderingMode:UIImageRenderingModeAlwaysTemplate];
UIButton *secondButton = [[UIButton alloc] initWithFrame:CGRectMake(45, 0, 35, 35)];
[secondButton setImage:secondButtonImage forState:UIControlStateNormal];
[secondButton addTarget:self action:@selector(secondButtonPressed) forControlEvents:UIControlEventTouchUpInside];
UIView *leftBarItemsView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 80, 35)];
[leftBarItemsView addSubview:firstButton];
[leftBarItemsView addSubview:secondButton];
UIBarButtonItem *leftBarItem = [[UIBarButtonItem alloc] initWithCustomView:leftBarItemsView];
[self.navigationItem setLeftBarButtonItems:[NSArray arrayWithObject:leftBarItem]];