You have your litre to gallon conversion wrong. There are 3.78l in 1US gall, so-
20l=5gall. Minimum recommended for a group of females is 10gall, that's without the neons.
10l=2.5gall. Pushing it really as a minimum size for 1 betta.
5l=just over 1gall. Really too small
Dividing the 20l tank for 2 bettas would be OK, but don't add neons.
It's probable the dropsy your female died of was caused by bad water quality. Are all these tanks filtered and how long have they been running?
20l=5gall. Minimum recommended for a group of females is 10gall, that's without the neons.
10l=2.5gall. Pushing it really as a minimum size for 1 betta.
5l=just over 1gall. Really too small
Dividing the 20l tank for 2 bettas would be OK, but don't add neons.
It's probable the dropsy your female died of was caused by bad water quality. Are all these tanks filtered and how long have they been running?