Before I get into it, I want to highlight for you three articles I think you should read on this subject. Medieval Silkwork has 2 articles on Supportive Underwear - in Visual Sources and in Written Sources. The third article is By My Measure's post On Cleavage and Breast Mounds.
Option 1: Non-Supportive Chemise + Supportive Cotte
I think it's safe to say that this method is generally accepted as the ideal method as indicated in the period sources. That's not to say that it's the accurate or more acceptable method- just that it's what we see the most. That's definitely a pro in its favor. I recently outlined this combination in part one of my look at early 15th century French clothing, and I do believe that it's an elegant solution.
Option 2: A Supportive Chemise + Supportive Cotte
We can find several allusions or indications that support built into the chemise may have been a thing a woman might do on an individual basis. Women with larger busts were being told that containing her breast was not only fashionable but also decent. So a woman with a considerable bust (such as myself) may have looked at and taken advantage of all opportunities to add support and containment to her clothing. In this scenario, support is located in both the chemise and the cotte.
The largest advantage to this is that, if the cotte were to fail for any reason, the chemise would be there to literally pick up the slack. And since a chemise is never meant to be a public garment, crude adjustments can be made to the fit of a chemise if the body's shape changes enough to warrant them.
The major point against this method for myself is that there is little to no visual evidence for supportive undergarments in early 15th century France. Hence, no images in this section.
I have decided to move forward with option 2. This is what I have been doing for a number of years already, and I haven't had a problem with that. The added security of two layers of support instead of one makes me feel comfortable and confident. The important thing now is to remove the crutch of bra support, and do a better job of creating solid, supportive fit with just these two layers.
I've begun this process by preparing a muslin toile based on my current symmetrical pattern. There's no real need to start from scratch here with a brand new body block since my current pattern is functional but also already loose and ripe for adjustments. Starting with it removes a lot of the initial fussy drape patterning. It's my plan to create both my supportive chemise and my cotte layers with this muslin pattern when the time comes.