Don't mix them. Dogs seem to like new kibble, so if you give one and then a different one and then go back, he'll pretend he likes them all because they're new each time. I'd give a few of one, see how he reacts (swallowing them whole is a good indication sometimes) and then wave the next sample by his nose (don't let him eat it, just tease) and then give another of the first sample. If he loses interest, the second might be better... The other way is to hold both firmly in separate hands and let the dog sniff them well and see which hand he tries harder to open.
At this point, my doggies know when they're going for a taste test, so I just have to hold them up and say "which one?"...
