Difference between 给 and 跟 (e.g. in 我给你讲 and 我跟你讲)?
給 = Give/Let, 跟 = With/Together. When used with another verb together, "給" designate the "transceiver" / "receiver" of the verb, while with "跟", both party are the subjects.

The examples you chose is a indistinguishable case (there are probably a lot of them). Let me elaborate with something else:


我給你吃飯 = I let you eat
我跟你吃飯 = I eat with you
你給我吃飯 = You let me eat
你跟我吃飯 = You eat with me / You and I eat together
我給你休息十分鐘 = I give you 10 minutes to rest
我跟你休息十分鐘 = I rest for 10 minutes with you
你跟我休息十分鐘 = You rest 10 minutes with me / You and I rest for 10 minutes together
我給你看看 = I'll show you this / (I give you a look of this)
我跟你看看 = Let's see this / (I look at this with you)


As for your examples, I guess we can distinguish them literally with:


我給你講 = I give you a speech
我跟你講 = I speak with you


However, the physical actions of these two are probably not distinguishable from each other.