Not much info out there on the par output of this light but I found a bit.
Here's the info we do know.
View attachment 202411
So 56w and you have 2 units.
One Czech website makes this claim about the 48w 1200 light.
"Every manufacturer measures intensity differently and it's very confusing. We measured different lights: Zetlight WRGB ZP4000-1200P according to our exact measurements has 4400 LM, and in water at a depth of 40 cm with one light measured 113 PAR."
Nové vodotěsné LED osvětlení řízené chytrým telefonem s plným spektrem. Verze podporující růst rostlin. Ve světlu je již zabudovaný přesně programovatelný stmívač s časovačem.
www.profiplants.cz
Can we take this reading at face value? Probably not because they're trying to sell the light, but without any other sources of info we'll use it just for the fun of it.
Obviously your light is the higher wattage 1500, for the sake of argument let's assume it has the same 113 par output at 400mm, but wait, your light is 730mm above the plant. Fear not, we can use the inverse square law and try and guesstimate what it would be at that distance, using this calculation which I'll be honest is a tad above my head we end up with a figure of about 33 par, add in a bit of overlap because you have 2 lights and maybe we can hit 40 par directly under the light.
These numbers are complete speculation of course, I've made loads of assumptions, used a par reading that may or not be accurate, and tried to do a calculation that at best isn't exact when it comes to light travelling through water and bouncing off glass walls and then there's my limited understanding of the inverse square law 😀
How does this help answer your question. Well if the number of 40 par at your substrate is anything like correct then yes that's enough light to grow alternanthera sp of plants, it would probably be considered in the low end of light intensity, but still enough to comfortably grow them.
My sums could be well off, so take them with a pinch of salt.
What I can say with some confidence is that alternanthera can be grown in a low tech tank without high light levels. Will they be the beautiful specimens that we see in some tanks, maybe not.
Here's some alternanthera grown low tech 1200mm long tank with 1 standard fluval aquasky light (14.5 W) 450mm from the substrate, guesstimate by me about 20 par. Not perfect, but alive.
View attachment 202414
Like the others have mentioned, plants fail for lots of reasons, lack of light should be fairly low down on that list.
… in any event, always very good matter-of-fact posts that we always learn from. Solid and dependable! 
