Have open angle glaucoma in 1 eye & may switch from travatan to cosopt. Will side effects of cosopt be less because I only need it for 1 eye?

Probably yes. One eye treatment with Cosopt at twice daily with punctal occlusion should have less systemic absorption than both eyes twice daily. Ordinarily if you are healthy without pulmonary contraindications (asthma, copd) or heart contraindications ( heart block, sick sinus syndrome) or other ( major depression ) or sulfa allergy just to name some, then you should tolerate well. It does sting a bit.
Only your eye doctor. Has all the necessary data about your glaucoma to be able to assess risks and benefits of the different eye drops esp in the setting of your overall medical health. If you wish to have another opinion find someone fellowship trained in glaucoma.

Related Questions

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?

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. Read more...
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. Read more...