This all sounds romantic, if a bit morbid.
You say you are looking for a wife or a husband, yet your husband is straight. So if you "brought in a 3rd to share" that was male, he and your present husband would not be sexual partners, just good friends at most. If he needed to lean on this guy for support if you died, how would that be different than leaning on any of his other guy friends or brothers that he has? Both men would've lost their wife.
You assume they would both then rather quickly find a new wife for them to share, to replace you? How is this different from your scenario where a widower remarries the first woman that comes along? What if they both instead got their own girlfriends or wives? Maybe one would want a new gf sooner, and the other took longer to get over your death and didn't want to date any other women for longer than the other guy?
Do you expect them to go out looking for women with the line, "We are both widowers of the same woman, would you be interested in taking her place?"
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
me: Mags, 59, living with:
miss pixi, 37