Is it not both your 2nd homes regardless? If you're both selling properties to fund the purchase/renovation? Or am I missing something.
If you sell one property and buy another in a linked transaction ( what we normally call being in a chain ) then at no point do you own two homes.
The issue here will be who owns which houses at which point.
You currently each individually own a house.
If you both sell prior to buying, you can put both names on the deeds for the new house with no issues. You won't pay the higher rate of SDLT for buying a second home, nor will either of you pay CGT for selling a second home: because neither of you will own a second home at any point.
If one of you sells one house before buying the new one but the other only sells their original house AFTER buying the new one then either:
A) You put both names on the new house, pay the higher rate of SDLT ( and attempt to claim it back ) and also pay CGT on the later sale of the other original house, because the non-seller will own two homes for a certain period; or
B) you only put one name on the new house (the person who sells their own house in advance) and don't add the second name until AFTER the second original house is sold. This means there is no need to pay the higher rate of SDLT nor any need to pay CGT, because neither person at any point owns two houses.
Your other option is to keep both existing houses and rent them both out. You would pay higher rate SDLT on your new purchase and would not be able to reclaim it, but you would not pay CGT because you wouldn't be selling anything. You would have to check the tax implications of receiving a rental income ( there's a threshold over which you start getting taxed ) to see if the numbers stack up, and you would have to pay CGT if /when you finally did sell one of rental properties... But houses are in massively short supply so will continue to rise in capital value and there's been an exodus of private landlords from the rental market ( loads selling up, loads more switching to holiday letting only because the tax system is more beneficial and the holiday let customers have no tenancy rights whereas long-term tenants do. ) so you could theoretically make a tidy income. Having said that, if you are in an area of high holiday letting demand you will make more money going down that route.
Source: used to work in conveyancing, have old law school buddies who still do.