Are two different premixed human insulin (70% NPH / 30% soluble) brands interchangeable, and is it okay to switch between them?

Insulins with the same composition and mix ratio are generally regarded as equivalent, but insulin is one medicine where I'd really discourage switching on your own. Even small differences in how a brand is manufactured can shift how your blood sugar responds, especially in the first few days after a change. Please loop in your doctor before switching, so your dose can be rechecked if needed.