1. A single character cannot weild two weapons at the same time. (Well...except for (SPOILER)Reddas, but that doesn't count. Plus, spear weapons are two-handed weapons.

2. I don't think so. I may be wrong, but I think that when you get the Zodiac Spear from the Necrohol of Nabudis, the chests you weren't supposed to open disappear. I'm not entirely sure, but I think that's right, because I went back to the Phon Coast after getting the Zodiac Spear, and one or two of the chests were gone; I didn't open any of them on the Phon Coast just in case (which I why I think they disappeared after getting the spear).