My understanding is that the first parents IHT allowance can be used when the second parent dies, but only if the first parents estate has been left to the spouse, not children?
Its not a case of bundling the estates together, but adding the first IHT allowance to the second one, giving a total allowance of 650K.
From the money advise service website:
They can also pass on their unused tax-free allowance to their spouse. For example, if a husband dies and leaves his entire estate to his wife, his wife can take his allowance and add it to her own tax-free allowance. This combined allowance means that when she dies, her estate will only incur Inheritance Tax if it’s worth more than £650,000 (£325,000 + £325,000).
I may be wrong, and it may still apply in your case, but it doesnt sound like it from the piece above. Maybe give the probate helpline a call, they are v helpful.