Depending on type of mantella they do climb, the aurantiaca and pulchra can be kept quit simmilair in conditions.
When kept to warm they will die from 'heat-related-muscle-spasm-syndrome', altough it's mainly the aurantiaca you have to be carefull about.

Mixing both species might seem to be a good idea because of this, but you better keep them seperated.
The Pulchra are quit nervous and the aurantiaca is rather bold, this may cause the pulchra to stress out.
Not to mention the different skin secretions and possibility of cross breeding.

I would use the high tank for the aurantiaca since they are more secure and will use the open climbing space.
For the pulchra you do best by providing floor space with lots of hiding spots.
In both tanks a large amount of leaf litter is something that can't be missing from the tank.