They stain differently because poplar is much softer than maple. You need to stain the maple and then get some pieces of scrap poplar to play with. I could tell you more but I don't want to go to the trouble if you don't want to take the time and expense and effort. If you don't, get the minwax stain and pratice on a piece of scrap maple and poplar.

Basics of staining.

Here is a link that might be useful: Staining wood

Thank you both for the info, my original idea is probably too ambitious for my level of experience. I think I'll just stick to ordering all unfinished cherry pieces and hope that I can get those to match fairly well. Thanks again, great link, Jody

In the hopes that you still answer responses to this post, I wanted to ask you which manufacturer produced your kitchen bay window. I have been looking for a quality wood bay window and yours appears to be just that, or it may be fiberglass clad with wood. Mind giving me just the simple rundown of what you remember about it. Thanks!
jay

The darker the better in terms of being able to disguise the difference. The overglaze/toning will also help. Staining real cherry dark seems like such a waste to me.
Casey