I've kept both species so here a lil pro and contra list:
merumontanus:
+ very small, don't need much space, only round 20cm long
+ livebearers
+ in relation to their size giant horns

+ could be sometimes housed together
- their climate is not so easy:really deep temperatures at night and on day never more than 25°C for a longer period
quadriocornis:
+ could be definitly housed together
- need much more space than merumontanus (males could be 40 cm long !)
+ have maybe the most variable coloration
- egg layers, with little clutches (round 5-12 eggs if I remember right)
+ have also beautiful horns and fins
- they need also very low temperatures
Hope I can help you a bit. Both species need an perfect water supply and prefer it to be housed outdoors.
Cheers
Benny