Many things we don't know, like the size of the venue, number of warm bodies, ambient noise level, dancing or listening.
The next question is, will the cost of an additional sub be a burden? If the answer is no, read on.
We have to make a couple of assumptions. You have determined that you do need the second full range speaker to feel confident your audience is getting the quality performance you are working to deliver and, you have doubts that the single sub will deliver the feel / volume of the bottom end to cover venues you have been playing.
Lets explore some reasons for having the second sub.
Ease of setup, no separate stands needed.
Balanced appearance, never hurts to look good.
Even though bass is omnidirectional, still probably easier to get a even, balanced sound with two.
If you play heavy bass content music you can move more air which can provide more "feel", add extra punch to a bass drum.
If loud or heavy bass is not necessarily your goal you can still provide more even coverage of a space at lower volume with two subs.
I think by doubling the subs, you gain 3db in volume, but in my opinion volume never tells the whole story when talking about bass.
Last, you have a wider range of possibilities, meaning you can crank it up, turn down, use one, or two, match to the venue, and the needs of a particular gig.
What is the most important thing you gain with two subs? Peace of mind.
Now if you list your reasons for not buying the second sub, we'll help you decide.