Um why? Give him half, let him decide if he's comfortable or not and then give him the other half. Why would you give him half, then give him the other before the first even kicked in? He won't know if he's had too much or too little yet.
LSD as much as I love it, is certainly not for everyone and if the boards you have are indeed potent, half will definitely produce effects. I'm an experienced user and I had a half a potent board of the weekend and it did me nicely.
If it's redosing you're concerned about, in my experience redosing works up to 2, 2.5 hours after initial consumption. Not to mention if they're really potent, unless you're already coming down a redose will be more than noticeable. Don't forget if he likes the experience he can always buy more and up the dose at another time, there's no good reason to dive head first into an extremely powerful hallucinogenic substance, but I can think of a couple of good reasons not to. There's plenty of time to explore the rabbit hole.