[Perasite]

http://www.emulamer.com/6700kbfix.html
That links to a program that allows for multiple key presses.
Quote from his site:

HTC, in their infinite wisdom, decided to write a keyboard driver that intentionally allows only one key to be pressed at a time on the keypad of the 6700. In my opinion, the device is almost perfect as a portable gaming unit- small, 320x240, overclocks nicely, and landscape with a keyboard that has more than enough keys for most emulatable systems. So, I get the thing as soon as it's released, charge it up and install Morphgear. To my horror, I discover that only one button will work at a time. Eventually, I get over the that the device was needlessly cripped in this manner, and start looking for ways to fix it. Not wanting to write an entirely new driver, I ended up with a program that works for alot of games and doesn't need to replace any system files. Basically, it does the job by intercepting messages sent from the keyboard driver before they get to the applications. After getting a message that a key's been released, it checks the keypad matrix registers on the CPU to determine if that key really was released, or if it's HTC's driver trying to ruin my game of mario world. If it's the latter, the message is discarded before it can reach the application.

First, note that I've only used this myself. I haven't had any problems with it what so ever, but I haven't tried it in conjunction with all of the other software out there, and I haven't tried it on the XV6700, although it should be the same. Also, this is somewhat hardware specific to the CPU and keyboard layout of the device, so I would imagine that trying to install this on another kind of device at best wouldn't work right, and at worst, may cause problems enough to hard reset it.

I've used it and it works great.