Not heard of that per se but we've recently re-done our wills (properly this time and not DIY
) and we have a "survivorship clause"
"If my husband the said xxxx not survive me for 30^ days, but in such event only, I direct my executors to make over the residue of m yestate to such of my son yyyy^, residing at...."
Like you, we're leaving everything to each other but then to ds.
We've also included a "common calamity" clause whereby if all of us have died, our respective estates go to SIL.
Plus we have double "failing which" executor clauses: each other, failing which SIL, failing which my best friend's dh.
A couple of reasons for this: my dad had a family friend as well as the lawyers as executors. Unfortunately, she died a few months before him, so it has just been the lawyers acting as executors, which has caused problems (issues between sibling and me, not dealt either well by the lawyers
) and also, should a "common calamity" occur to dh, ds and me, I don't want a single penny going to my then next-of-kin, who would be my sibling
Dh also has a similar issue with one of his other siblings.