I have 2 valid passports but the one containing my Non-Imm rentry permit is getting full so I wanted to transfer it to the second. They are saying this is not possible without cancelling the 1st (nearly full) passport. Even if it gets full, they won't put a new permit in the new passport until the old passport is cancelled (corner cut). That defeats the purpose (quite legitimate) of having 2 passports. Anyone else experienced this or know if/how it is possible to achieve the transfer without cancelling the 1st passport (or declaring it as lost here...). Thanks.

Nope, I had the same issue, they won't transfer anything to a second passport without you cancelling the 'from' passport.

Since it's full you'll be needing to get a new PP anyway so just wait until you apply for a new PP before attempting the transfer again.

NOTE: Assuming it's a UK passport you now need a justification letter from your employer when renewing any of your multiple UK passports.