If you are duo-toning with selenium and gold, the selenium (if you limit to the darkest areas) will 'warm up' the shadows and stop the increased blue-black dMax you get with gold toning (be careful not to print too dark initially, especially if you're not selenium toning, but gold toning only). You have to be careful not to tone in selenium for too long, if you keep it to the shadows, the gold can work particularly effectively on the mid-tones, giving a great sense of recession. The highest highlights become brighter - maybe too bright or light, if you don't want this, and/or if you are not careful with the initial printing. So you might want to print a little 'flatter' than you would normally. (If there is no tone available, either on original print or because already toned in selenium, of course the gold will not take effect, and there will not be a colour shift).
Timing the selenium can be tricky, but the gold is easy because it works slowly (and you don't have to worry about it spreading too far if you've already part-toned in selenium) - I find for a visible blue effect it can take about 40 minutes.
There's a picture of an orchard in my gallery & portfolio which is selenium/gold toned, both the brands you are going to use, printed on Ilford Warmtone. Bear in mind the colour varies according to monitor display, as I found, some people couldn't see any blue at all, and probably with others it is quite blue! The print has quite a discernible, but subtle colour shift, but the greatest effect is to brighten the print (which is nice for a snowy scene) and at the same time give greater depth. Also the developer/developing method can effect the colour shifts (I found there is a slight but discernible pinky tone with this combination with Agfa NE).
Always wash carefully between stages and Tim Rudman suggests fixing after gold toning is preferable if not always completely essential. And yes, don't let prints overlap either in the toning stage or washing.
edit: should add that although gold toner generally works quite slowly, if you use it after sepia, the colour can change quite rapidly, through a range of colours, yellow/orange/red, so you have to watch it more carefully for the colour you want.