Please explain the checks in more details.
Thus, anyone who creates a block may create a non-standard transaction, but that transaction -- once it makes it into a block -- must still be verified by the same basic checks that guard against incorrect signatures, double-spending, etc.
I doesn't see incorrect transactions in my scenario. Miner with new version validates transaction to the cleint with new version and it is perfectly valid transaction with good inputs.
Old clients will refuse to relay sendmany transactions
So it is possible for miner with old version to confirm the second transaction. And that transaction will be perfectly valid for him.
(I assume, that old client rejects the particular transaction, but acceps the block chain from the new miner. Otherwise there will be a chain fork)
Which basic checks will not pass ?