Gonna add some more to consider.
First. Don't get too carried away with lighting.
When I was first looking into plants. I was under the impression that a bunch of light was required to grow plants fast enough to out grow algae. And the cool side affect of using up nitrates.
The bunch of light isn't required. Above average, yes. But we can go over board with lights, real easy.
40w's over a 20 L, is alot of light.
A 20L is one of those tank "sizes" that messes up the WPG rule.
Once you get a substrate. The bottom of your plants are only about 9" from the surface. Opposed to a reg 20g, that would be 13" from the surface. 4" of water is alot, when it comes to light penatrating. Ask anybody with a 55g, about how much faster their plants grow when they get 6" from the surface.
When you stick a stem plant ion your 20L, its already going to be with in 2" of the surface. Most you buy will already be 7-9" long. And under 40w, at 2" below the surface. You could be trimming 2" off the plant every 4-5 days.
I haven't heard anything yet, about plants you already have or the kinds wanted. But just wanted to give an example of how things could go in that size of tank. And while on examples. Anacharis in that set-up could easily grow an inch a day.
This looks like a good time to ask...
What plants are you wanting?
Do you already have plants, that aren't growing?
Or
Is this just for more viewing, light?
While we're waitng on that...
Back to the lighting.
If you go with:
Glass top
Use your existing fixture
Add one more single fixture
You can run each light seperate.
So, if you start having algae problems. You'll be able to do stuff like, run both lights for 6 hrs. And leave the other on for an hour or so on each end, for your viewing pleasure.
If you get a double light fixture. Both will come on and go off at the same time. So your only option is total on/off. Which would be a bummer when cutting into your viewing pleasure time. Then it turns into an, all about the plants/algae. Instead of the "viewing pleasure" that we typically start a tank for.
This is another good reason why smaller tanks are harder to deal with, opposed to the standard 55g type tank.
The 20w over the tank just isn't enough. But 40w is on the verge of too much. As in, the option of low or high light. Not really much of a medium, using standard off the shelf equipment.