I might be able to help.
I always described my wife as doubly incontinent as that is how she would have been described medically, and the continence specialist to whom we were referred assumed that this would now be a permanent symptom of the condition. Urinary incontinence usually strikes first and those who believe in 'the stages of dementia' put it in the later stages and remark that late stage dementia often involves double incontinence.
I've always thought it odd that for Sue things developed the other way round. Like so many people, Sue had a longstanding tendency towards constipation. So when, four years ago, she started to have soiling problems, we used any number of laxatives and other drugs which various medics swore by. They turned out to be worse than useless. They simply complicated the issue and, whatever dose of whatever drug you used, you never really knew if there was any progress. Sometimes there seemed to be, but then you were suddenly back to square one.
Fast forward: For about a year now, the problem has almost disappeared. Sue has empties her bowels nearly every day and nearly all the time 100% goes down the pan. After months when there were absolutely no 'accidents' I now find some poo on the pad when she wakes up about once a fortnight. This only happens after she has had a day when she hasn't 'performed', but doesn't always happen after such a day. Believe me, after our previous experience I find I can cope with this without batting an eyelid.
The improvement hasn't been achieved suddenly of course. There have been false hopes in the past but overall things have gradually improved over time.
How has this been achieved? So these are my tips, mostly discovered by trial and error:
1) Regular exercise, e.g. we aim to walk at least 5 miles a week whatever the weather and we usually exceed this target.
2) Vegetables are very important. We tend to eat a lot (we don't eat meat). Some vegetables are extremely effective - carrots, sweet potatoes, squash, onions all work well for Sue so I concentrate on these.
3) Insoluble fibre, particularly wheat bran and anything that contains it, has to be treated with caution as it can have exactly the same unwanted effect as the drugs.
4) Soluble fibre, in particular oat bran, is great and much of the improvement can be traced back to when we started to have porridge for breakfast - real porridge not instant refined muck.
5) We discovered that immediately after the evening meal is the optimum time (for Sue) for a successful visit to the loo.
6) Before every meal I give Sue fruit finger foods - mango, melon, apple, pears. I've always understood the importance of fruit but this fairly recently introduced tactic means that Sue eats more fruit than she did before.
7) Obviously, it's also important to keep hydrated and this is sometimes difficult - Sue has to be watched to avoid spills and she sometimes drinks very slowly.
I'm sure none of the above is particularly novel. And what has worked for Sue may not work for others but I hope that they may be of some use to someone.
I've realised that it is very important, if you embark on or develop such a regime, to hold your nerve. One dose of any kind of laxative can screw things up for a considerable length of time. Bin them!
I also avoid some foods. Raspberries, which Sue used to have every day because she liked them, were a definite culprit, as I eventually realised.
N.B. I am not suggesting that this precise regime will suit everyone. I think you have to do your own experimenting. But, believe me, it's worth it. it is no exaggeration to say that it has vastly improved our day-to-day lives.