Sticky little problem

Hey all, I've recently planted two plants pretty close together, just to see how it'd work out.
So here's my problem, one of them is dying, and I'm not sure whether it's because I've over fertilized (it's not going yellow) or planted too close together and one of them is suffocating the other one.

It looks like the edges of the leaves are burning basically, turning black and falling off.
Anyone know what's happened?

Thanks man, I'm thinking its probably the soil, but I'll let them grow and see how it turns out.