This video will show you how to charge two batteries using one deltran battery charger junior. If the two batteries are exactly the same make and model and state of discharge, you can charge two at the same time, but if they are different batteries, you can only charge one battery at a time.
And I've also shown a way to make a quick disconnect plug from common household items so that you can easily yank and go and not worry about damaging your connector on your battery tender as you throw the connector on the floor.
The battery tender connectors are made of a soft metal it seems, and if you remove and disconnect the battery as often as I do, there is the possibility of wearing it out.
Not including stuff that I have lying around this build cost only 85 cents.
The only specialized item that you might not have are the alligator clips, however, if you wanted to have alligator clips, you could buy smaller simpler ones from an electronic shop, or even simpler, just make a cheap permanent quick disconnect attachment to that battery.