Yes and know. With a completely dead battery, 6 or 12V you will need a high charge rate to get the process started. By that I mean something over 50 amps. Most chargers can't do that. I have hooked up a fully charged battery (same voltage) to a dead one with jumper cables and then put a charger on at the highest rate. Sometimes works as you have described. Glad you were able to save your batteries.
2016 Wildwood 28RLDS
2014 Tundra 1794 Platnum,"23" T-Bucket
2011 Harley Softail, 2010 Spyder RTS