Using un-updated 3.0e material
Prior to the “v.3.5 revised edition,” critical effects usually stacked, which made it quite possible to achieve a 2-20 critical threat range. This was one of the big things they changed in the revision. Almost everything that affected critical threat range was explicitly updated to nix stacking.
However, under the rules for using 3e material in a 3.5e game, the disciple of Dispater prestige class from Book of Vile Darkness remains legal and un-updated. Its iron power ability triples threat range, and explicitly stacks with Improved Critical (but not keen). This is (arguably, at least) more specific than 3.5e Improved Critical’s statement that it doesn't stack with anything, since iron power addresses Improved Critical specifically.
Realistically, though, the guidelines for 3e content in 3.5e call on the DM to update un-updated stuff. So in practice, they wouldn’t stack.
With an 18-20 weapon, iron power achieves 12-20 threat without Improved Critical, or 9-20 threat with it. However, there are a couple of 17-20 weapons out there (stump knife from Arms & Equipment Guide, which has all kinds of limitations, and bladed gauntlet from Sword & Fist, which does not but was errata’d to 19-20), and there is also the streetfighter barbarian ACF which increases critical threat range by 1 when charging or attacking a flat-footed foe (and that’s straight 3.5e material). Combined, a stump knife with streetfighter, Improved Critical, and iron power achieves a critical threat range of 1-20, though of course you still have to hit, which you won’t on a 1.
As discussed in this answer, a targeteer fighter from Dragon vol. 310 can use the sniper ability to trade attacks for critical threat range increases. Get enough attacks—probably using natural weapons—and you’ll reach 2-20. Iron power will definitely help you get there more easily, even if updated to not stack with Improved Critical.