Is it ok to trickle charge 24volt batteries with a 12 volt charger or should i disconnect the bridging cable first
wont harm the batteries but might not do your charger any good, like you said, disconnect them first.
To charge a 24 volt system you need an input of at least 28.8 volts to beat battery impedance
12 volt charger connected to both batteries connected in series will burn out the charging transformer/rectifier
Easy way is to disconnect 24 volt link and connect positive to positive and neg to neg with a pair of jump leads and charge both batteries in parallel.Just connect charger to pos and neg as normal and charge both batteries at the same time
Both batteries will then get full charge off one 12 volt trickle charger.