Check the offerings from Comet/Maldol and Diamond antennas. They make any number of after-market dual-band HT antennas. The best would be one that is a half-wave or 5/8 wavelength long on 2 meters and stacked 5/8 wave elements on 440, but this would be 3 feet long or so. So you probably want something shorter. There are several that are a quarter-wave on 2 and 5/8 wave on 440; these are anywhere from about a foot to 19 in. long and will still stick you in the armpit if you wear your HT on your belt. Most hams just go with a dual band antenna that is partly or fully wound into a coil and covered with rubber. They average about 7 inches but some are shorter. The stock antenna is of this sort. The more of the radiating element that is wound into a coil, the less effective the antenna but the easier it is to carry: the more it's stretched out, the better the antenna but the less convenient it will be on your belt.
Bob, WoNXN