Red and purple

I think the colours would look cute together, how do you plan on placing them? One colour at the roots of the hair and one at the ends?

I would dilute both dyes with some conditioner so that they have less run off and wash them off as separately as possible when rinsing the dye out of your hair to stop them affecting each other. If they do run in to each other and stain the other colour a bit it won't matter too much as they wont make a horrible shade.

I agree those colours would look lovely together! Howver, be aware of directions plum. While it makes a lovely purple at the start, even when diluted a lot (it will come out super dark applied neat), it will turn blue quite quickly and that blue is nearly impossible to get rid of. So either put the plum on the very ends if you want to try it out and don't mind getting a cut if you don't like how it develops, or maybe try a purple that's more pink based? Since cerise is pink too.