AS bd6759 says, no nomination is possible.
OK, thanks for the clarification.
I got myself confused after reading the following on a different site.
The above rules only treat the property as being occupied as a residence – not necessarily as a main residence. Therefore, if you have another residence during the period (whether owned or rented) and you wish to secure main residence relief on the property from which you are absent, it is recommended to make a nomination to HMRC confirming that you wish to treat that property as your main residence during the period. If you do not make such a nomination, the question of which property was your main residence will be decided on the facts.
https://www.litrg.org.uk/savings-property/capital-gains-tax/selling-your-home#5
I don't know the answer to this - but it doesn't really matter any more although it used to be important:
https://www.legislation.gov.uk/ukpga/1992/12/section/222 (Make sure you look at the latest version, not the original version)
F5(5A)But a notice or further notice under subsection (5)(a) determining which of 2 or more residences is an individual’s main residence for any period may be given more than 2 years from the beginning of the period if during the period the individual has not held an interest of more than a negligible market value in more than one of the residences.
So in your case, if both properties are residences, then you can backdate an election as long as is necessary.
I think the problem is that your second home might not be a residence. See this article:
https://www.markmclaughlin.co.uk/main-residence-election-is-the-property-a-residence/
The problem is that there's no statutory definition of residence therefore to be sure you will either have to find a case with conditions similar enough to your own to rely on or, should you need to claim PPR for more than 9 months after you moved, wait to see if HMRC objects and then appeal.
The sort of case I'd look for would be one where someone had two residences (undisputed), they decided to sell the one that they'd made a main residence election for but it took a long time to sell. Did HMRC try to disallow PPR for the period it was being marketed on the grounds that it wasn't a residence any more? I think there's a vast difference between continuing to be a residence after quality of occupation changes, and becoming a residence originally but I don't know whether that's ever been taken to tribunal. Once a property is a residence the quality of the occupation doesn't matter for PPR election purposes.
FWIW, when I was in a somewhat similar situation, my buildings insurance required the property to be visited *every* week (it was was more onerous than if it had been a main home - missing a week due to being on holiday wasn't allowed) although I wasn't looking to make the property my main residence for CGT purposes so it's not directly comparable but that would seem to strengthen the claim to be a residence. I sold it after the then 18 months and I squeaked in under the CGT allowance once the PRR apportioning was done.