Actually when you play dundun, you play 2 (or more) instruments.
The drum and the bell. Each instrument have their own rhythm, but in the end, when they reach your ear, they are already mixed into one polyrhythm.
So the short answer is: both approaches are correct if you can only deliver the final effect. If you think you cannot then you need more practice to grow the muscle memory. It will help your hands keep playing what they know well while you focus/do/think of something else - be it drums or not.
Polyrhythms and polymeters
West African music is full of polyrhythms and polymeters.
In the traditional dundun style, the combined melody of dundunba, sangban and kenkeni (and bells) is usually a polyrhythm - notes (or whole phrases) are more likely to alternate than overlap. You won't hear any polymeters, triplets or even notes shorter than 8ths in the dundun part. These techniques are very popular with djembe or other solo drums though. Some dundunfolas use them when playing a dundun set.
Let me share a few techniques I use to improve my feeling, memorize patterns and fight the constant feeling of being lost in this crazy music that I can't stop falling in love with.
Feel the beat
Memorize melodies in reference to the beat. Always. This means pretty much what you described as playing the metronome hits with one hand and the actual melody with the other. Or you can just sign your melody, vocalize it, and clap the beat with your hands. Dancing to it can help too. Once you can sing it, it means your brain has learned and now it can teach your hands. Otherwise you can end up learning the wrong melody without even realizing (until you come to play with other people).
Learn the bell
Many rhythms share the same bell pattern. Others can sound quite similar, but it's important to know the difference so you won't get lost in time. When you learn a few key patterns then you can... that will be said later.
The pattern you've mentioned
x-xx-xx-x-x-x-x- is one of those popular and good to know. It hides a little challenge in hearing the second beat in the pause between the double stokes. Others worth mentioning are:
x-x-xx-x-x-x you can learn in both 6/8 and 4/4 feelings
-xx-xx-xx-xx worth spending a lifetime, dunumba style (6/8)
xx-xx-xx-xx- e.g. soko sangban (6/8)
x-xx-xx-xx-x e.g. soko dundunba, soli (6/8)
- drum stroke is always supported with the bell stroke
- all pauses are single 8th notes
- avoid triple strokes
So you play only 8ths: either a single note stroke, a double stroke and there is an 8th note pause between each stroke.
F...orget the rules
And learn the triple stroke
x-xxx-xxx-xxx-xx pattern too. It may be quite challenging at higher tempos.
Forget the bell
You can also take a more relaxed approach and let the bell follow your melody naturally, filling in the gaps to match your drum strokes. This is much more effective when you already know some patterns, otherwise you may end up playing metronome (there's nothing wrong with that, but you may find it hard to come up with another pattern quickly unless you've memorized some).
As you have already realised, playing just one mixed melody is much more viable, but it comes at a cost. When you make a mistake it will probably knock out your both hands. Also it feels somewhat harder to improvise/variate when you operate on the combined melody.
Finally I don't think it is possible to develop fully independent hands (excluding those playing 40h/day), but I can assure you constant practice (and only this) is the key to improvement. At some point your muscles will learn how to play patterns without the constant supervisory of your conscious mind. You will be able to remind yourself (the more you practice the more quickly) and then play a sequence of bell-drum patterns just by thinking about the first few notes. Think of typing on your phone with text suggestions on. This is your language. Patterns are the vocabulary.