Perhaps you didnt plant it deep enough? Or could one be getting water and one too dry?
Try removing all the dead stuff and cutting it down to the ground, give it a good water if dry and apply a thick mulch, with any luck it will recover by next year.

Any idea what varieties they are? If the one with brown leaves and only a few flowers is an earlier flowering variety, but the healthy one with masses of flowers is a later, viticella type, that might explain why they look so different currently.

They are same type, planted a the same time, ill go have a look and see if it looks like they are planted at different depths

If they're both planted in the same way, then there's either something dodgy about the poorly clematis itself, or some difference in the conditions for one compared to the other, as Drc suggests - less rain, more rain, less sun, more sun, close to large roots of another plant, something like that.