Do you need one for day and another for a longer trip or hiking backpacks the same? If you are just going on a day hike, you just need one large enough to carry what you can’t stuff in your pockets. This type of pack has a top load compartment with a zipper flap. Inside you could stuff a jacket, flashlight, picnic lunch, first aid, binoculars, wash cloth, and most have a mesh bag on the side for a water bottle. And it usually has a couple of smaller pockets that you could put a phone, keys, or a camera in.
Now, if you are going on a journey. You need a lot larger backpack that can carry a few more essentials. You need to match your pack to your activity. If your climbing you will have more weight to carry, because of ropes, tools, etc. So, you may want a narrower pack, so you don’t get snagged up on rocks or limbs. A top loader is a little larger than a day pack and closes with a draw string. And you will want a hydration reservoir. Also your going to want to think about load levelers, straps, padding and comfort. A man’s pack may not be comfortable for a woman. So, do you need two hiking backpacks or can you make do with one kind?