Use travatan open angle glaucoma in 1 eye & eye is bloodshot. If I try cosopt is there a risk of vision loss during the trial period if doesn't work?

Unlikely. Glaucoma usually cause visual damage over a period of months to years. It is unlikely that a therapeutic trial with Cosopt would lead to visual loss. Cosopt is frequently more effective than travatan but realize that is consists of 2 separate drugs combined in one bottle.
No. Vision loss in glaucoma (except for the rare acute glaucoma) is very slow. Most drops have some degree of effectiveness for several days after discontinuation. And the onset of effectiveness is rapid. So you are describing a situation in which your coverage needs will be effectively met and there is no risk of vision loss.