Hi Sarah,
The Place of Supply Rules don’t really go into any detail around this but I can confirm from previous conversations with HMRC, they would expect the work to be apportioned and the invoice to reflect that. In your situation, you’re acting for two executors where one is in the UK (subject to UK VAT) and the other outside of the UK (Outside the Scope of UK VAT) so the invoice/bill should be reflect that and be split i.e. 50% taxable at standard rate and the other 50% as Outside the Scope of UK VAT. I’d recommend you issue two separate invoices in this situation for clarity.
VAT is always fun – NOT!
Hope this helps