I would say a great deal of the possible friction would be the level of adherence of each to their stated beliefs. Also, the differences between the denominations.
For example, my MIL was raised baptist and was a member of a standard (in GA) Southern Baptist church when she married her 3rd husband (the first husband was baptist, the second was Catholic, the 3rd was Jehovah’s Witness). She was not overly devout and it seems she was more into the guy and not being alone than she was concerned about the differences of faith.
I am not sure how devoted he was to JW prior to their marriage. By the time I met my wife, he was only an occasional attender. But I know my wife said she had to attend some of their services and bible studies. She was 13 when her mom married him and she rebelled. The two denominations are very different. SB consider JW a cult and JW considers non-JWs as not being real Christians.
He refused to let them have a Christmas tree (no that is not thing for Christianity, but there is also no problem with it), so his very strict adherence to his beliefs, did cause friction, but over time, he gave in (probably for the nookie). So by the time I met them (10 yrs later), he had relented on most things and they just did not talk about religion. In fact, we got married in the church that she and my wife were members of (SB) and he came to the wedding and it was not a big deal.
My wife and I are devout SB and it works nicely for us, if one wanted to convert, there would be a lot of conversation as to why.
For example, when our oldest was a freshman in HS, she said she was BI. There is not a lot of acceptance for such in the SB church, so there was a possibility that it could cause issues and would potentially enough of a reason to change denominations (we would not attend somewhere our DD was not welcome).
But we would have converted to something close but also was more open to the LGBTQ community (Methodists for example).
Were one of us to suddenly decide to become JW, or Mormon or Christian Scientist, then that would likely cause a lot more issues and could fracture our marriage. But considering our involvement in the church and our ongoing communication in all areas, that would not happen without lots of prior discussion and reflection.
So theoretically, it could be a problem, but in practice, I am sure we would head it off long in advance. If there was no communication, then it would be due to one of us changing not just our beliefs/practices, but something very dear to our core. That is ultimately what would cause any issues.