I helped a friend with his skidsteer today.

It has two 12V batteries in series, but it is a 12V system, so it seems to me the batteries must be charging to 6V. Is that a problem? I think he would be better off with them in parallel.

Also, something odd: my Schumacher charger/ starter would not give power if I connected it across the two. ( connections are good).

It would only charge when clamped to a single battery. Maybe the charger somehow sensed the multiple batteries and didn't like it?