Need info on battery pack

I took a 36volt 12ah agm lead acid battery and wired in series to a 12volt 12ah agm battery to get a 48volt 12ah pack to power my 48volt 1000 watt rear hub ebike coversion kit. I was getting a 36 volt kit but ended up with a 48 volt kit and a 36 volt battery . so I ordered a 12volt battery to add to it instead of ordering 4- 12volt batteries to wire together since I had already bought the 36 volt battery. I rode the bike 10 miles and it went 30 miles an hour so it works but when charging is it going to get charged unevenly or is the 12volt battery going to fail earlier than the 36volt battery.? Or is this ok to wire 2 different voltage batteries together? the 36 volt battery is smaller than 3-12 volt batteries combined so it saves space in the battery pack on the back of the bike.
Thanks in advance for some insight on this.