I'd read Haste as saying "One extra attack with any one weapon you hold."
But Haste has other benefits that Speed weapons don't grant. Bonuses to hit, to AC, to Initiative, to Reflex saves. So they aren't really equivalent.
Yeah, Speed won't stack with Haste, and if you could somehow place two Speed enhancements on a single weapon I wouldn't let them stack. But two separate Speed weapons, each granting you an extra attack with that one weapon? I don't see that as stacking because the effect is specific to the one weapon, not to you. You don't get to choose which weapon you get the extra attack with.
YMMV, of course, but that's how I'd handle it.