Two subs are generally better than one depending on.........
Two REL Strata are better than one, but I cannot say for sure that two REL Q108E ($750) subwoofers are as good as one Strata ($1500); or that two Strata are better than one REL Stadium which is what $3000 will get you. You get better bass for the price, so two of a lesser model may not be the way to go.
The sub at the very least has to be the equal or better than your speakers. No good using a cheap sub with $3000 speakers.
However, if you bought one Strata now because $1500 is your budget, and saved up for the second later, that would be a good scenario. The Strata is less $$ if you get black brittex color.
So I would get the best single sub I could afford, and decide later if I really want a second one. You may decide later that you're happy with one.
The subwoofer will work a little better if it only has to play the bass of one channel over two, and depending on how your room is, you might get better results having two subs at opposite sides of the room instead of one in a single place. Low low bass is much less directional than a speaker, which is why most people just get one. Low bass is felt, not really heard. You may or may not notice enough difference to justify the expense. But those with two obviously like it.
I have a REL Storm III. I did use it in my main music system until I got new and much better (read expensive) speakers. I like the bass of the new speakers without the sub, so I moved the Storm to my separate movie setup. The movies sound more realistic with the Storm over the old less expensive sub that was there. So a double happy ending :-)