First, the N900 only has 1 antenna assembly for cellular, and one for Wifi/Bluetooth -- while I haven't looked into it deeply enough to be certain each of those isn't two separate antennas, their size is much more appropriate for a single antenna on their respective frequencies. (Publicly available info should suffice to confirm one way or another, so don't take my guess as gospel.)

Second, even if you had a two-antenna system, with the received signal strength from both exposed to the user, indications would likely have more to do with how you're holding it and nearby objects than with the direction to the signal source. Would still be cool, but not very useful without a tripod or similar to mount it with minimal nearby disturbances.