Couple of questions:
1) What OS are you going to be running to take advantage of dual PIIIs? If it isn't NT or Windows 2000 you're gonna be wasting a great deal of money for processors that get there ass kicked (mhz for mhz) by your present, single Athlon 1GHZ.
2) What software will you be running that can take advantage of dual processors? I would guess that there are very few mainstream applications/games/etc. where you will see enough of a performance gain to justify the extra money.
Things to think about.
If you're really worried about the Slot A deadend, IMHO you'll get much better value/performance from a KT133A board and socket A. Heck, for less than that money (for dual PIIIs and MB), you can probably have a 133mhz board with DDR, a 266mhz FSB Thunderbird, and 256MB of PC2100 DDR ram! (a setup, which, by the way, will be kicking the P4's ass probably well into 2002)