Lots of good points being made here Colin.
Split sepia toning will work (via bleach) top down, tonally, i.e. it starts with the highlights and then midtones leaving shadows to last. You can stop the bleach where you want and tone only part of the tonal range, giving a colour contrast between light and dark tones.
The bleach will always reach down a bit further than you can see, so the sepia colour will usually extend further down the tones than you might have thought when watching the bleach. Pull from the bleach a touch on the early side. You can always take it on a bit by re bleaching (care - it will appear to go faster) but you can never take it back.
Warm tone papers bleach faster than colder tone papers. You may need to dilute the bleach more (quite a bit) to get control and reproducibility with them.
Make sure that you wash (and if FB, use hypo clear) all fixer out well before bleaching, or you will lose tones irrevocably.
Tone fully in sepia toner bath, don't snatch. It will only tones what you have bleached.
If you selenium tone as well, ST works bottom up - the opposite to sepia. This is helpful.
If you ST MGWT paper the dark tones will soon shift into brown as has been pointed out. this gives you brown at both ends, which gives the shadows depth but diminishes the obvious 2 colour effect as the 2 colours are now browns, albeit different browns.
If you tone many WT papers in ST, there is a point where the blacks cool off a bit before warming up to brown. You may need quite dilute ST for this (papers vary). This is nice for duo-toning with sepia. You can run a set of test pieces processed to max black and ST for 1,2,4,8,mins etc to see when this happens, BUT if you use this time guide on a print that has been split sepia toned the blacks will shift to brown much quicker! So be aware.
If you choose to ST first and then sepia, remember that ST has fixer in it which you must get out before bleaching! As has been pointed out, it is harder to see how far the ST has gone, which is why it can be easier to do sepia first. But if you use sepia just to highlights this shouldn't be a problem as they take a while to convert in ST.
A good point was made about a dilute post-toning stop bath, to get rid of the scummy deposit, especially in hard water areas.
A hardener in the fix is rarely needed these days as papers are mostly robust. It can interfere with toning. You could use a de-hardening bath.
Water 750.00ml
Sodium carbonate, monohydrated 30.00g
Water to make 1000.00ml
Soak prints for 10 minutes
Best wishes
Tim