Hi Jorge

I often use glycerin development for my platinum print. In fact I use different product so to keep thing simple I prefer to use a new fresh developper for each print. When I do a series I sometimes use a bath developer for it . But it often happen that in one priting session I print different seies print and then I use glycerin development.

I've rarely done split development but it happen that on some image I get the same problem as you talk about or I want to mask a print and do it chimically by using different concentration of developer (1 glycerin-1 PO vs 3-1) or different developer (1Gl -1 Po vs 1Gl - 1 Citrate Pot.)

so let's answer your questions.
No you want get any uneven development , for this you need to well brush for the first 30 sec -1minute depending on the temp after you just need to brush every minutes. ( it also depend on the temp of the dev, hot quicker , colder slower)

Using to developper is a little harder but this depend on the image ,if there is a sky line , it will be easy. But if there are trees coming from the dark and going to the sky you should take care of the developper.
In this case I prefer to first apply a first layer of pure glycerin and then apply the developper glycerin mix.

If you work with the same dev. with different contrast thing will be easy. I start with the high contrast or low contrast developper depending how big they are , I always start with the bigger zone starting from the top o it and getting to the developer limit , then I use the other developer.

I don't do it often but I 've never get visible marks, just need to get al around your prints , 2 or more brushes (large and small), dilute developer in glycerin and pure glycerin.

I will try to rent a digital camera to show the way I work , sometime images explain more than words.