They are different techniques with different outcomes and objectives Paul.
The split grading is really IMO to quickly get both the exposure time and contrast grade (filtration) matched to the negative. From there you can interpret the final print the way you want it without having to cope with unnecessary dodging and burning from having the contrast & filtration mismatched (what you might call rescue dodging/burning, as opposed to interpretative or creative dodging/burning).
It also gives you the option of dodging during either the Gr.)).) exposure or the Gr.5 exposure, or a combination - thus altering local contrast too.
Flashing is useful to raise the threshold of the paper to exposure so that it is much more 'sensitive' at the highlight end, where little light is getting through. This makes burning in either unnecessary or much easier when it is required. It also alters contrast and so can be used with graded papers for in-between grades.
The two techniques can be used separately of together as required.
Various flashing tools have been discussed here. The RH flasher has its own timer, which can be useful. It is commonly used on the enlarger though, near the lens. I find this less convenient as its distance from the paper - and hence the flashing time - changes as the enlarger is moved. I like to get a 'max-flash' time for a whole box of paper, which I can then repeat without re-testing for diferent enlargers. I also found that blu-tacking it onto the base plate tending to cause alignment shifts on my enlargers so be aware and avoid using pressure to hold it in place - elastic bands might be better. I find it simpler to use a second enlarger for flashing or a distant low wattage night light (I use both for different applications) - connected to my timer (I use the second channel on my R H Designs Stopclock Pro for this - very easy to toggle back and forth)
Another point to be careful of: If your flasher is not exactly in the negative light path (e.g. a distant flasher, or one alongside the lens or lens board, it will shine obliquely on the paper and if the paper is in the easle you will probably see an unflashed 'flash shadow' strip along one or two sides on the image. Don't flash in the easle unless you can exclude this.
Tim