not heard of this before,
i can only think the 'danger' was with the old oriinal batteries, if one battery is heavily discharged it will suck power in from the more charged battery at a detrimental rate to cable an the charged batt.
but with car batteries we often jump start in effect the same way, so if one battery is flat the current draw from a fully charged sister battery would not be unhealthy to the wiring or the charged battery.
by not connecting simultaneously, it would have the same effect surely?
Alan