I don't remove the canvas to pin, it is just easier for me to pin on the frame. My belly bar has both leaders too. I have (rarely) taken a quilt off to frog when there was a lot to do. I could pin to the canvas off the frame. I find that with the heavy canvas I don't have to pin as closely to make it secure. My Canvas leaders are 8 feet long, the striped ticking part is the full 10 feet
of the frame. This leaves me room (1foot) on each side to change the bobbin/clean etc. I haven't ever had trouble with this aproach. When floating I adjust the batting and top with each turning of the takeup bar. I sew down the side (whichever is closest) and quilt to the other side , then sew down the far side to secure it when I get there. I use side clamps on the backing when I remember to. I hope this makes sense.