Poison and splash do combo, though admittedly splash probably should have been more powerful on its own
The problem with the splash poison combo is the you are still FORCED to use one of the 3 attribute slots on direct damage.
If you could get the flask to do AOE damage, poison, and heal or shield, it might be more useful.
Also, since, from what I can see, there is a max of 2 enemies, the AOE is sorta wasted, if you could splash poison 3+ enemies it would be cool, but since the moment the first enemy dies the aoe becomes useless, it's just not worth it.