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If that van are anything like my friends 92 F250 (and like my 90 van), then I'd say the two batteries are connected together (+ to +). I forget the correct term (parallel or series). It's wired this way to increase the AMPS to crank I believe. Both batteries SHOULD be identical (CCA's and type-dry or wet- NOT one of each). That's what I've been told.
I've learned over time when changing batteries on tractors using two batteries on 12V systems (not 24V), that if one is bad, the other will be soon. Especially if both were installed the same. On a rare case, one might be good for a long time, but w/ diesels, do you want to risk it? If they are not that old, you might get away w/ replacing one. I've seen people do it w/ big semi's when they have 4 batteries and one goes bad. But usually, it's best that all are replaced. You might get a spare out of it (install in a gas rig... I do that and get another year or so out of them or longer).
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1999 E-350 Cub Wagon, 7.3L Power Stroke, E4OD, 3.55 
1990 E-350 Club Wagon, 7.3L IDI, E4OD, 3.54LS
1983 F-150 2x4, 4.9L, C-6 w/GV-OD, 3.55 Farm pickup
1981 C-8000, 3208 CAT, RT-6510, Rockwell SSHD Tandems (Swap)
1977 K100C, NTC-350, RT-1110, Tandem
1977 Transtar II, NTC-290, RT-9509 Single
1977 Transtar II, Formula 290, RT0-9513, Tandem
1974 C-750, 391CID, Clark 5 speed, Eaton 2 speed
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