Normally, no.

You can have both connected to the same computer at the same time (if you have two network cards), but the operating system will only use one connection at a time.

You can get around this by using 3rd-party software that will perform “connection teaming”, essentially combining two Internet connections to work as one extra-fast connection. However, such software is hard to find. Reportedly, the Vicomsoft InterGate software has this feature, but I couldn’t find a consumer version of InterGate on their website (even though I know such a product once existed). I also found a product called MidPoint that includes connection teaming as part of its Internet sharing functionality, but it looks like the program hasn’t been supported for several years; the website for the company has been taken down. You can still download MidPoint, but there’s no guarantee that it will work on Windows XP (or even do exactly what you want to do).

Good luck, and let me know how it goes.