Clever, clever trick.

C4WorkSpace.m

#import "C4WorkSpace.h"
#import "ClassA.h"
#import "ClassB.h"

@implementation C4WorkSpace

-(void)setup {
    ClassA *objA = [ClassA new];
    ClassB *objB = [ClassB new];
    
    [objA methodA];
    [objA methodB];

    [objB methodA];
    [objB methodB];
}

@end

ClassA.h

#import "C4Object.h"
#import "DefaultProtocol.h"

@interface ClassA : C4Object <DefaultProtocol>

@end

ClassA.m

#import "ClassA.h"

@implementation ClassA
-(void)methodA {
    C4Log(@"%@ | %@", NSStringFromSelector(_cmd), NSStringFromClass([self class]));
}

-(void)methodB {
    C4Log(@"%@ | %@", NSStringFromSelector(_cmd), NSStringFromClass([self class]));
}

@end

ClassB.h

#import "C4Object.h"
#import "DefaultProtocol.h"

@interface ClassB : C4Object <DefaultProtocol>

@end

ClassB.m

#import "ClassB.h"
#import "ClassA.h"
#import <objc/runtime.h>

@implementation ClassB

+(void)initialize {
    Method methodA = class_getInstanceMethod([ClassA class], @selector(methodA));
    
    class_replaceMethod([self class],
                        @selector(methodA),
                        method_getImplementation(methodA),
                        method_getTypeEncoding(methodA));

    Method methodB = class_getInstanceMethod([ClassA class], @selector(methodB));
    class_replaceMethod([self class],
                        @selector(methodB),
                        method_getImplementation(methodB),
                        method_getTypeEncoding(methodB));
}

-(void)methodA{
    C4Log(@"a");
}

-(void)methodB{
    C4Log(@"b");
}
@end

DefaultProtocol.h

#import <Foundation/Foundation.h>

@protocol DefaultProtocol <NSObject>
-(void)methodA;
-(void)methodB;
@end

Output.txt

Output WITH implementation copying
---
[C4Log] methodA | ClassA
[C4Log] methodB | ClassA
[C4Log] methodA | ClassB
[C4Log] methodB | ClassB

Output WITHOUT implementation copying
---
[C4Log] methodA | ClassA
[C4Log] methodB | ClassA
[C4Log] a
[C4Log] b