Can I share my binder between my iPhone and ipad?
Indeed you can! Go to Settings & Tools at the bottom of the Main Menu. Open Tools at the top of that menu. Half way down hit Back Up to Dropbox. Then on your other device, follow the same process except hit Restore Collection and Restore Decks instead.
This is assuming you have a Dropbox. If not, set one up! Very beneficial not just for this app but everything else digital.
So to reaffirm for quick reference:
iDevice 1:
Settings & Tools> Tools> 2nd Backup All option (Dropbox symbol)
iDevice 2:
Settings & Tools> Tools> 2nd Restore Collection option (Dropbox symbol)> 2nd Restore Decks option (Dropbox symbol).