I have an ACS400, but cannot find a manual for it.

I am confused by its two inputs, Input 1 & Input 2.
There is no switch to choose one or the other.
Are they left & right audio inputs?

I assume these are passive speakers. Are the two jacks pass throughs? (Stereo phone jacks) Connect one speaker to the source and connect the two speakers together with a stereo jumper. You could prove this with a flashlight cell. Connect the jumper, “thump” one channel, then the other.

There is an explanation right here:
[Link: assets.hardwarezone.com]

It is a computer speaker/amplifier system with a powered sub-woofer. All inputs come in through the right speaker.

Thank you. I have seen that article before. It implies that

1. both inputs are independent stereo inputs and
2. the "center" potentiometer just sets the volume ratio between the two.

I normally expect the center pot to adjust the ratios of L & R audio.

NO! That is not what the diagram is showing and telling. Look closer.

The 2 inputs are = 1) Front left & right. 2) Rear left & right.

There are 2 sets of drivers in each left & right speaker cabinet. The front firing 3" drivers for the main Left & Right channels. And a second set of 3" drivers on the side, that are for the rear Left & Right surround channels.

It is meant to be hooked to an old PC computer (ex. 2009) as the source, which had 2 sets of stereo jack outputs (Green = Front Left & Right and Black = Rear Left & Right).
Many modern motherboards still have a similar analog output these days but with an additional Orange = Center & Subwoofer output to it. Although color coding vary a lot now.

Thanks for the clarification. I am using it as TV speakers that does not provide surround sound.
So, a conventional front-only configuration would be input #1 only?
How should the center pot be set if there is no input #2?

I can not find the original manual for those speakers anywhere online. And the diagram & wording in that link is contradictory on what exactly the center level is doing.
Either way, since you are using a TV (I'm guessing the headphone output or some line level output), it will be controlling the volume. So just set all the level controls on the speaker at about 80% and enjoy.

Thank you.