There are actually two answers to your question. The first is yes, it's normal for the mains to be coming in when there is sufficient throttle opening to allow for enough signal to begin drawing fuel from them. The second, it no, it's not normal for them to be working at idle! Even if your idle is a bit on the high side, you should not be pulling fuel from the mains! What this means is that you are having to open the throttle too much to achieve desired idle, and you're bypassing the idle circuit and transfer slots. This is something that needs to be corrected, and can cause alot of problems. Throttle response, cylinder washdown, diluted oil, and other things come to mine. If the carb is correctly calibrated for your combo, and everything is adjusted right, you should have about .020" of transfer slot showing on both primaries and secondaires (if this is a mechanical secondary carb that is), and the idle mixture screws should be out from about 1-2 turns. You should be able to achieve the correct idle speed with small adjustments to the mixture screws, not the throttle plates. It's also common for the throttle plates to have small holes drilled in them for the purpose of bypassing air to help with idle.
Anyway, hopefully this will help you out some. Like I said, I'm no expert, and am still learning myself, but have gotten alot of information from people that know alot more than I do and I'm just trying to pass that info on to you. Let me know if I can help any more.