You mean the 2 x Play:5/gen2 set (“Two Room Pro Set”)? Yes, that appears to be treated as a ‘single product’ for the purposes of the discount.

Yes that’s the set I’m after sorry for being unclear

thank you for your reply I have also had a response for sonos customer care which says the following 

I can advise that at the discount applies to either one product or one set. For further information please visit these links below.

Trade Up

Terms and Conditions

I hope this information is of some assistance to you.

Thank you. 
Kind regards, 

David B

Sonos is being very nice, traded my old ZP 80 for a discount on a 5.1 Beam system and got the discount off the combination price.

I have a Gen 1 Play 5 that qualifies for the trade up program. First I would like to thank Sonos for such an offer , no other companies really do this. That said if I were to trade up I would like to go to a new play 5. Problem is Gen 1 came out in 2009 and eol at least for new software is May 2020. Play 5 Gen 2 came out Nov 2015. We are now 2020 so that makes it already about 4 and 1/4 years into its life cycle. Thus perhaps it would last another 7 years before it is slated for doomsday.  For this reason I won’t spend money at this time.  Sure I could buy one of the other speakers, but value wise my play 5 gen 1 still sounds great . It sounds better than the play one, it plays equally as well as the move does (IMO). Even has a carry handle  when I take it to my deck. My wish right now is that Sonos gives the Play 5 gen 2 some love in the form of an upgrade.  Then I may upgrade with confidence. Other than that it may be time for me to move on. 

Answered at your other post

https://en.community.sonos.com/wireless-speakers-228992/trade-up-scheme-6831609/index8.html#post16398537