Not if the second AP is on the same channel. Only one radio at a time can talk on the channel. This is true whether it is a second AP in your house or an AP in a neighbor’s house that is on the same channel (within range). If you set your second radio to the same SSID and passcode, with a different channel then you should be able to roam between them fine and you will not be cutting your bandwidth by using the same channel.
Correct.
Same channel, shared medium. Different channel, not shared medium.
You also have the additional issue if both are talking at the same time, you are going to have contention. So actually, you are likely to run in to a situation with two APs where you might of have 35Mbps with one AP and multiple clients connected to it, where you'll actually have reduce bandwidth with two APs and clients connected to both and trying to receive/transmit at the same time.
Because of contention. If the APs are physically seperated enough that the transmissions can't step on each other, no issues. For example in my setup (just talking about 2.4GHz), I have 3 APs, with AP 1 and 3 sharing the same channel (11). However, they and their clients are always going to be physically highly seperated. The closest overlap has a signal difference over >>>30dBm. In testing, file transfers on both APs at the same time from different clients have zero impact on each other (when the clients were connected to the appropriate APs).
My AP2 in on Channels 1+5, because its coverage DOES physically overlap with APs 1 and 3. If they shared channels there, there would be/is contention. Being on seperate channels though, there is no overlap or contention with the other APs or clients on those other APs.
It also means that instead of having to deal with hidden node issues and contention and all of that kind of stuff reducing bandwidth, I have a nice clean low interference setup with the benefit that I have 600Mbps of through put on AP2 (300Mbps 2.4GHz, 300Mbps 5GHz), 130Mbps on AP3 (2.4GHz only and 20MHz...outdoor AP) and 430Mbps on AP1 (130Mbps 2.4GHz 20MHz only, 300Mbps 5GHz) that I can potentially utilize if I had clients dispersed around my house/property, or 1160Mbps.
If I had them all on the same channel I'd have at best 600Mbps, minus contention issues.