... when I click on the request to sign message, it signs it with a different address (I have bytes in 2 different addresses). How can I sign the message with my other address?
What address is that? A change address or just another the wallets in the app? Just select the wallet you want to add from the burger menu and then select "Insert my address" from the chat "..." button. It is easiest to do that with single-wallet address because on multi-address wallets, you need to add a new address every time you spend from that wallet because change is sent to your new address. So, if you use multi-wallet address, it makes sense to send all to your current address on "Receive" screen.
I mean another address in the same wallet.
Is there a general interface that lets me sign any message, with any address that's in my wallet? The only interface I've seen is in Chat, when the bot sends me a "Request to sign" message, and clicking on that doesn't let me pick the address used to sign it.
I guess a workaround is to send everything to a single address, but I'm surprised that there isn't a general message signing interface.