No you cannot. The Beam Gen1 nor Gen2 cannot be used as a stereo pair under any circumstances. Only the following like speakers can be used as a stereo pair:

Current Speakers 
Sonos One with a Sonos One or Sonos One SL

Sonos One SL with a Sonos One SL or Sonos One

Sonos Five with a Sonos Five

Discontinued Speakers 

Play 1 with a Play 1

Play 3 with a Play 3

Play 5 (Gen2) with a Play 5 (Gen2)

Also there is no Sonos One SL Gen2 Duo speaker

so just to confirm:Can't you connect a beam 1gen with sonos one sl speakers to expand surround sound via the sonos 2 app?

Yes, the Sonos Beam will ‘bond’ (not ‘pair’, this is basically Sonos jargon) with the a pair of the One SLs to provide surround sound when both are using S2. 

I don't quite understand both answers...

can I now use a 1st generation sonos beam with sonos one sl gen 2 speakers?

in other words does the beam gen 1 support with sonos one sl gen 2?? I would like to know if they connect to each other and produce surround sound through TV as usual.

I hope this clears up your confusion. 

Yes you can use a Beam Gen 1 with two Sonos One SL’s as surrounds. Stop confusing things referencing Sonos One SL Gen1. It doesn’t matter. 

The Beam is a speaker that you attaché to your TV. You can then use two Sonos One SL’s as surround speakers. If you like you can also add a Sonos Sub for deep bass and low end effects for movies. The complete setup is called Sonos Home Theater.

In the end you have the Beam up front attached to your TV. The surrounds are at the rear of the room. The sub can be placed anywhere, typically in a corner. 
 

To conclude: you cannot “pair the sonos Beam (1gen) as a stereo pair with the Sonos one SL (gen2) duo Speakers” as you asked, but you can bond two One (SL) speakers as surrounds to a Beam.

The document linked below gives the full picture of compatibility.  From it you can see that the One SL requires S2 and that the Beam (gen 1) is compatible with s2 (and also S1, but that is irrelevant here).  As they can join the same system, they can be combined in all the standard Sonos room configurations, including soundbar with surrounds.

https://support.sonos.com/s/article/4786?language=en_US

Hi All,

From SONOS documentation I was also able to exclude which devices support the S2 app.

However, I was not completely convinced whether the "Sonos One SL" speakers are supported together with the Beam (1gen) for surround. I have now purchased the sonos one SL speakers and have been able to set them up for surround via the S2 App. In any case, I have been helped, thank you very much for your fast support.

Sonos has S1 and S2 software (and so the S1 and S2 apps) and also has Gen 1 and Gen 2 devices. The question if a speaker kan run S1 and S2, or just S1 or just S2 is not directly related to their generation. The Beam (Gen 1) can run on S1 and S2 software, while the Play 5 Gen 1 for example can only run on S1 software.

You have S2 capable devices, so S2 is the best for you.