Vet appointment?

One of my boys needs to go and get some blood work done with potential kidney issues and will have to stay in the vet all day due to him needing to be made sleepy for the blood to be taken. Will it be better to just take my boy that needs the work done or shall we take both? If we leave one behind then my partner will be home all day with him, so he will have company, just not his brothers company. They have never been apart in their whole two years of living. Separating them is just making this whole thing even sadder for me! Pfa of my sweet boys