I prefer to keep my toothbrushes constantly charged. This is something I do with my mobiles too.
just some advice on some types of rechargeable batteries:
Some types are not affected by being re-charged before they are fully discharged. However, these type of batteries are good for an approximate number of re-charging cycles.
So, if you use your battery until it needs re-charging, say for about a week, and it's rated for say 100 re-charging cycles. It'll last for about 100 weeks.
If you re-charge every day, regardless of it's state of discharge, it'll last for about only 100 days. Obviously the re-chargeable cycle is only approximate. Some, rated at say 100 cycles, may last for more than 500 cycles.
The other common type of rechargeable battery has a memory, and if you re-charge it when it is only half way discharged, it'll think it only has half its capacity for future re-charges.
So, in summary, you're best to follow John's advice and re-charge 'em only when they need it. The batteries will last longer before being unable to be recharged.
If both brushes are the same type, just use one charger and interchange them as and when needed.