I would be pretty surprised if you were able to reply from the forwarding number.
I really wish Apple and carriers made a multiple phone setup much easier and smoother. I would like to use two phones (using one number), but I’m too put off by things like not being able to send an sms or make a call from the correct number regardless of which phone I pick up, and having different text conversations and other things stored on different phones.
For me, I would use two phones for—a small main everyday phone, and a big secondary phone for certain occasions that I know I’m going to need more screen or battery or camera.