The two Symfonisk devices need to be the same type to stereo-pair them, but you can choose to ‘group’ them instead as shown in this support link:

https://support.sonos.com/en-us/article/group-and-ungroup-rooms

So I have to use two Symfonisk frames or two Symfonisk lamps for that to work as stereo?

Yes, the two speakers need to be the same acoustically to produce (matched) stereo audio output from each channel, so for stereo you need two alike (same) devices, but alternatively you can ‘Group’ different speakers to produce (usually mono) audio output, whilst they’re acoustically different, the grouped devices will still play nicely and in perfect sync. 

This is usually the same for every speaker manufacturer - and across the Sonos range, you cannot stereo pair a Sonos One with a Sonos Roam speaker for example, as the sound from each is quite different - you need the L/R stereo channels to acoustically ‘match’ each other, so you would use two Ones ‘paired’, or two Roams ‘paired’ for stereo listening, but you can still choose to group those two different speakers instead. The same applies to the Symfonisk range.

And not only stereo, the same applies for surround speakers.