I feel for you so much because we are in the same position with our two boys. We have come to terms that there probably will not be a reconciliation because they lead such different lives and have very different values. It doesn't help that only one son has children because children can help to heal situations like this.
The last time we were together was my husband's 70th birthday tea at our house, nearly four years ago, and it was very awkward. It was my 70th birthday a few months later so, like you, we did two separate meals out but it broke my heart. We were also unable to celebrate our 50th wedding anniversary as a family which would have been our wish but instead of doing the two separate celebrations we went away on holiday instead. No more double celebrations for us, as you say it is exhausting.
Although I am very soft hearted I can feel my heart hardening about the situation and as they are joint executors of our will we have recently told them that we will either instruct our solicitor to act as executor, which will cost us money to change the will and will cost them money as the executor will obviously have fees which will have to be paid from the estate, or be men enough to shake hands and get on with what has to be done together. The choice is theirs as far as we are concerned.
I think it goes with the territory of having sons, as my husband says, it is usually the wife who calls the tune and we know which wife has majorly contributed to the problem, although the other one is not without fault. Things were fine when my older son was married to his first wife and we were all able to get together and have good times.
I have a friend with two sons and they were in the same position until last year when my friend was hospitalised for a while; the two wives' paths crossed at the hospital and they decided to talk things over and now they are able to socialise together which my friend is overjoyed about. So there is always hope.
Until I joined this forum I felt such a failure over the situation with our sons and the fact that we are estranged from our two oldest grandchildren, but now I realise our problems are not unique which is very comforting. Part of my heart however is broken beyond repair and it was devastating to hear my six year old grandson say recently that he didn't know his Dad had a brother. I had mentioned something we had done together when our boys were small. Similarly, he and his seven year old sister don't know they have two half sisters.
As for the way to proceed, your situation is unique and dependent on the personalities of those involved and, if there are grandchildren, whether you want to run the risk of being estranged from them. Don't do anything in haste and tread carefully but it seems very unfair that you are unable to have your say. Is there anyone other than your other son who can act as mediator?
Two old sayings come to mind:
When they are babies they break your back but when they are grown they break your heart.
A son is a son 'til he takes a wife.
I sincerely hope that things work out for the best for you and your husband.